Green
— Spectral coordinates —
Wavelength		520–570 nm
Frequency		~575–525 THz
— Common connotations —
nature, growth, grass, hope, youth, inexperience, sickness, health, Islam, spring, Saint Patrick's Day, money (US), greed, and envy[1][2][3]
— Color coordinates —
Hex triplet		#00FF00
sRGBB	(r, g, b)	(0, 255, 0)
Source		sRGB approximation to NCS S 2060-G[4]
B: Normalized to [0–255] (byte)

Green is a color, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 520–570 nanometers. In the subtractive color system, it is not a primary color, but is created out of a mixture of yellow and blue, or yellow and cyan; it is considered one of the additive primary colors. On the HSV color wheel, also known as the RGB color wheel, the complement of green is magenta; that is, a purple color corresponding to an equal mixture of red and blue light. On a color wheel based on traditional color theory (RYB), the complementary color to green is considered to be red.^[5]

The word green is closely related to the Old English verb growan, "to grow". It is used to describe plants or the ocean. Sometimes it can also describe someone who is inexperienced, jealous, or sick. In the United States of America, green is a slang term for money, among other things. Several colloquialisms have derived from these meanings, such as "green around the gills", a phrase used to describe a person who looks ill.

Several minerals have a green color, including emerald, which is colored green by its chromium content. Animals such as frogs, lizards, and other reptiles and amphibians, fish, insects, and birds, appear green because of a mixture of layers of blue and green coloring on their skin. By far the largest contributor to green in nature is chlorophyll, the chemical by which plants photosynthesize. Many creatures have adapted to their green environments by taking on a green hue themselves as camouflage.

Culturally, green has broad and sometimes contradictory meanings. In some cultures, green symbolizes hope and growth, while in others, it is associated with death, sickness, envy, or the devil, or both, depending on the context. The most common associations, however, are found in its ties to nature. For example, Islam venerates the color, as it expects paradise to be full of lush greenery. Green is also associated with regeneration, fertility and rebirth for its connections to nature. Recent political groups in Europe and the West have taken on the color as a symbol of environmental protection and social justice, and consider themselves part of the Green movement, some naming themselves Green parties. This has led to similar campaigns in advertising, as companies have sold green, or environmentally friendly, products.

Etymology and linguistic definitions[link]

The etymology of the word "green" is related to nature and growth.

The application of terms to ranges of the visible spectrum are essentially arbitrary, cultural conventions. The notion of "green" in modern European languages corresponds to about 520–570 nm, but many historical and non-European languages make other choices, e.g. using a term for the range of ca. 450–530 nm ("blue/green") and another for ca. 530–590 nm ("green/yellow")

The word green comes from the Old English word grēne, earlier grœni. It is from a Common Germanic *gronja-, which is also reflected in Old Norse grænn, Old High German gruoni (but unattested in East Germanic), ultimately from a PIE root *ghre- "to grow", and root-cognate with grass and to grow.^[6] The first recorded use of the word as a color term in Old English dates to ca. AD 700.^[7]

Latin with viridis (and hence the Romance languages, and English vert, verdure etc.) also has a genuine term for "green". Likewise the Slavic languages with zelenъ. Ancient Greek also had a term for yellowish, pale green, χλωρός, cognate with χλοερός "verdant" and χλόη "the green of new growth".

Thus, the languages mentioned above (Germanic, Romance, Slavic, Greek) have old terms for "green" which are derived from words for fresh, sprouting vegetation. However, comparative linguistics makes clear that these terms were coined independently, over the past few millennia, and there is no identifiable single Proto-Indo-European or word for "green". For example, the Slavic zelenъ is cognate with Sanskrit hari "yellow, ochre, golden".^[8] The Turkic languages also have jašɨl "green" or "yellowish green", compared to a Mongolian word for "meadow".^[9]

Indeed, in the comparative study of color terms in the world's languages, green is only found as a separate category in languages with the fully developed range of six colors (white, red yellow, green, blue, black), identified as "Stage V" in Berlin and Kay (1969), or more rarely in systems with five colors (white, red yellow, green, black/blue). Many languages with a more reduced inventory of color terms have a single word for subsuming what would in English be called either blue or green, or more rarely black/blue/green or yellow/green/blue.^[10]

Thus, especially the distinction of green from blue is lacking in the traditional vocabulary of many languages.^[11] Modern languages have of course introduced supplementary vocabulary to denote "green", but these terms are recognizable as recent adoptions that are not in origin color terms (much like the English adjective orange being in origin not a color term but the name of a fruit). Thus, the Thai word เขียว besides meaning "green" also means "rank" and "smelly" and holds other unpleasant associations.^[12]

The Celtic languages had a term for "blue/green/grey", Proto-Celtic *glasto-, which gave rise to Old Irish glas "green, grey" and to Welsh glas "blue". This word is cognate with the Ancient Greek γλαυκός "bluish green", contrasting with χλωρός "yellowish green" discussed above.

In modern Japanese, the term for green is 緑, while the old term for "blue/green", blue (青, Ao^?) now means "blue". But in certain contexts, green is still conventionally referred to as 青, as in in blue traffic light (青信号, Ao shingō^?) and blue leaves (青葉, Aoba^?), reflecting the absence of blue-green distinction in old Japanese (more accurately, the traditional Japanese color terminology grouped some shades of green with blue, and others with yellow tones).

The Persian language is traditionally lacking a black/blue/green distinction. The Persian word سبز sabz can mean "green", "black" or "dark". Thus, Persian erotic poetry, dark-skinned women are addressed as sabz-eh, as in phrases like سبز گندم گون sabz-eh-gandom-gun (literally "dark wheat colored") or سبز مليح sabz-eh-malih ("a dark beauty").^[13] Similarly, in Sudanese Arabic, dark-skinned people are described as أخضر akhḍar, the term which in Standard Arabic stands unambiguously for "green".^[14]

In science[link]

Color vision and colorimetry[link]

Chlorophyll is responsible for the green color in plants.

The perception of greenness (in opposition to redness forming one of the opponent mechanisms in human color vision) is evoked by light which triggers the medium-wavelength M cone cells in the eye more than the long-wavelength L cones. Light which triggers this greenness response more than the yellowness or blueness of the other color opponent mechanism is called green. A green light source typically has a spectral power distribution dominated by energy with a wavelength of roughly 487–570 nm.^[15]

In additive color devices such as computer displays and televisions, one of the primary light sources is typically a narrow-spectrum yellowish-green of dominant wavelength ~550 nm; this "green" primary is combined with an orangish-red "red" primary and a purplish-blue "blue" primary to produce any color in between – the RGB color model. A unique green (green appearing neither yellowish nor bluish) is produced on such a device by mixing light from the green primary with some light from the blue primary.

By contrast in process color printing, a subtractive color system, green can be produced via a mixture of cyan and yellow ink, and in traditional color theory, green is produced by mixing yellow and blue paint.

Green is complementary to a purplish red or reddish purple color, in both additive and subtractive mixtures, and in simultaneous contrast effects and afterimages.

The sensitivity of the dark-adapted human eye is greatest at about 507 nm, a bluish-green color, while the light-adapted eye is most sensitive about 555 nm, a yellowish-green color.^[16] Human eyes have color receptors known as cone cells, of which there are three types. In some cases, one is missing or faulty, which can cause color blindness, including the common inability to distinguish red and yellow from green, known as deuteranopia or red–green color blindness.^[17] Green is restful to the eye. Studies show that a green environment can reduce fatigue.^[18]

In minerals and chemistry[link]

Emeralds come in many shades of green.

Many minerals provide pigments which have been used in green paints and dyes over the centuries. Pigments, in this case, are minerals which reflect the color green, rather that emitting it through luminescent or phosphorescent qualities. The large number of green pigments makes it impossible to mention them all. Among the more notable green minerals, however is the emerald, which is colored green by trace amounts of chromium and sometimes vanadium.^[19] Chromium(III) oxide (Cr₂O₃), is called chrome green, also called viridian or institutional green when used as a pigment.^[20] For many years, the source of amazonite's color was a mystery. Widely thought to have been due to copper because copper compounds often have blue and green colors, the blue-green color is likely to be derived from small quantities of lead and water in the feldspar.^[21] Copper is the source of the green color in malachite pigments, chemically known as basic copper(II) carbonate.^[22] Early painters would also use copper in the form of verdigris mixed with wax and turpentine to create green pigmentation in paints.^[23] Mixtures of oxidized cobalt and zinc were also used to create green paints as early as the 18th century.^[24] A more complete list of green minerals and pigments can be seen here.^[25][26]

Fireworks typically use barium salts to create green sparks.

There is no natural source for green food colorings which has been approved by the US Food and Drug Administration. Chlorophyll, the E numbers E140 and E141, is the most common green chemical found in nature, and only allowed in certain medicines and cosmetic materials.^[27] Quinoline Yellow (E104) is a commonly used coloring in the United Kingdom but is banned in Australia, Japan, Norway and the United States.^[28] Green S (E142) is prohibited in many countries, for it is known to cause hyperactivity, asthma, urticaria, and insomnia.^[29]

To create green sparks, fireworks use barium salts, such as barium chlorate, barium nitrate crystals, or barium chloride, also used for green fireplace logs.^[30] Copper salts typically burn blue, but cupric chloride (also known as "campfire blue") can also produce green flames.^[30] Green pyrotechnic flares can use a mix ratio 75:25 of boron and potassium nitrate.^[30] Smoke can be turned green by a mixture: solvent yellow 33, solvent green 3, lactose, magnesium carbonate plus sodium carbonate added to potassium chlorate.^[30]

In biology[link]

Frogs often appear green because light reflects off of a blue underlayer of chemicals and through a yellow upperlayer, filtering the light to be primarily green.

Green is common in nature, as many green plants use a complex chemical known as chlorophyll during photosynthesis.^[17] Chlorophyll does not absorb green light because it first arose in organisms living in oceans where purple halobacteria were already exploiting photosynthesis. Their purple color arose because they extracted energy in the green portion of the spectrum using bacteriorhodopsin. The new organisms that then later came to dominate the extraction of light were selected to exploit those portions of the spectrum not used by the halobacteria.^[31]

Animals typically use the color green as camouflage, blending in with the chlorophyll green of the surrounding environment.^[17] Green animals include, especially, amphibians, reptiles, and some fish, birds and insects. Most fish, reptiles, amphibians, and birds appear green because of a reflection of blue light coming through an over-layer of yellow pigment. Perception of color can also be affected by the surrounding environment. For example, broadleaf forests typically have a yellow-green light about them as the trees filter the light. Turacoverdin is one chemical which can cause a green hue in birds, especially.^[17] Invertebrates such as insects or mollusks often display green colors because of porphyrin pigments, sometimes caused by diet. This can causes their feces to look green as well. Other chemicals which generally contribute to greenness among organisms are flavins (lychochromes) and hemanovadin.^[17] Humans have imitated this by wearing green clothing as a camouflage in military and other fields. Substances that may impart a greenish hue to one's skin include biliverdin, the green pigment in bile, and ceruloplasmin, a protein that carries copper ions in chelation.

Lasers[link]

Lasers emitting in the green part of the spectrum are widely available to the general public in a wide range of output powers. Green laser pointers outputting at 532 nm (563.5 THz) are relatively inexpensive compared to other wavelengths of the same power, and are very popular due to their good beam quality and very high apparent brightness. The most common green lasers use diode pumped solid state (DPSS) technology to create the green light.^[32] An infrared laser diode at 808 nm is used to pump a crystal of neodymium-doped yttrium vanadium oxide (Nd:YVO4) or neodymium-doped yttrium aluminium garnet (Nd:YAG) and induces it to emit 281.76 THz (1064 nm). This deeper infrared light is then passed through another crystal containing potassium, titanium and phosphorus (KTP), whose non-linear properties generate light at a frequency that is twice that of the incident beam (563.5 THz); in this case corresponding to the wavelength of 532 nm ("green").^[33] Other green wavelengths are also available using DPSS technology ranging from 501 nm to 543 nm.^[34] Green wavelengths are also available from gas lasers, including the Helium-neon laser (543 nm), the Argon-ion laser (514 nm) and the Krypton-ion laser (521 nm and 531 nm), as well as liquid dye lasers. Green lasers have a wide variety of applications, including pointing, illumination, surgery, laser light shows, spectroscopy, interferometry, fluorescence, holography, machine vision, non-lethal weapons and bird control.^[35]

In culture[link]

In the 15th century "Saint Wolfgang and the Devil" by Michael Pacher, the Devil is green. Poets such as Chaucer also drew connections between the color green and the devil.^[36]

Nature[link]

In many folklores and literatures, green has traditionally been used to symbolize nature and its embodied attributes, namely those of life, fertility, and rebirth. Green was symbolic of resurrection and immortality in Ancient Egypt; the god Osiris was depicted as green-skinned.^[37] It is often used to describe foliage and the sea, and has become a symbol of environmentalism. Someone who works well with plants is said to have a green thumb or green fingers, and the word greenhorn refers to an inexperienced person.^[2] A company is said to be greenwashing if they advertise positive environmental practices to cover up environmental destruction.^[38][39] Green is used to describe anyone young, inexperienced, or gullible (probably by analogy to unripe, i.e. unready or immature, fruit).^[1] Green was the traditional color worn by hunters in the 19th century particularly the shade called hunter green. In the 20th century most hunters began wearing the color olive drab, a shade of green, instead of hunter green.^[40]

Love and lust[link]

Stories of the medieval period further portray it as representing love^[41] and the base, natural desires of man.^[42] In Persian and Sudanese poetry, dark-skinned women, called "green" women may be eroticized.^[14] The Chinese term for cuckold is "to wear a green hat."^[43] It is because in ancient China sex worker's husband wears Green Headscarf.^[44] Green is also used to describe jealousy and envy.^[1] In more modern times, the consumption of green M&M's has earned urban legendary status as a purported aphrodisiac.^[45]

Death, decay, and evil[link]

Green is also known to have signified witchcraft, devilry and evil for its association with faeries and spirits of early English folklore. It also had an association with decay and toxicity.^[46] Actor Bela Lugosi wore green-hued makeup for the role of Dracula in the 1927–28 Broadway stage production.^[47] A green tinge in the skin is sometimes associated with nausea and sickness.^[48] A physically ill person is said to look green around the gills.^[2] The color, when combined with gold, is seen as representing the fading of youth.^[49] In the Celtic tradition, green was avoided in clothing for its superstitious association with misfortune and death.^[50][51] Green is thought to be an unlucky color in British and British-derived cultures,^[52] where green cars, wedding dresses, and theater costumes are all the objects of superstition.^[53] Spider-Man villains were often colored green to represent a contrast to the hero's red.^[54] In some Far East cultures the color green is often used as a symbol of sickness and/or nausea.^[55]

Prosperity[link]

The United States one dollar note, like all other American dollar bills, is historically green.

In areas that use the U.S. Dollar as currency, green carries a connotation of money, wealth, and capitalism, because green is the color of United States banknotes, giving rise to the slang term greenback for cash.^[1] One of the more notable uses of this meaning is found in The Wonderful Wizard of Oz. In this story is the Emerald City, where everyone wears tinted glasses which make everything look green. According to the populist interpretation of the story, the city’s color is used by the author, L. Frank Baum, to illustrate the financial system of America in his day, as he lived in a time when America was debating the use of paper money versus gold.^[56] Green can communicate safety to proceed, as in traffic lights.^[2] In China, green is associated with the east, with sunrise, and with life and growth.^[57] In Thailand, the color green is consider auspicious for those born on a Wednesday day (light green for those born at night).^[58]

Nationality and politics[link]

Sovereign states with green flags:

  Islamic states using green

  Pan-African colors (red, black, and green)

  Other, most commonly to represent either lush national vegetation or heraldry

Several countries use green on their flags for symbolic or cultural reasons. Green, for example is one of the three colors (along with red and black, or red and gold) of Pan-Africanism. Several African countries thus use the color on their flags, including South Africa, Ghana, Senegal, Mali, Ethiopia, Togo, Guinea, Benin, and Zimbabwe. The Pan-African colors are borrowed from the Ethiopian flag, one of the oldest independent African countries. Green in these cases represents the natural richness of Africa.^[59]

Many flags of the Islamic world are green, as the color is considered sacred in Islam (see below). The flag of Hamas,^[60] as well as the flag of Iran, is green, symbolizing their Islamist ideology.^[61] The 1977 flag of Libya consists of a simple green field with no other characteristics. It was the only national flag in the world with just one color and no design, insignia, or other details.^[62] In the run-up to Iran's 2009 presidential election, the reformist candidate Mir-Hossein Mousavi chose green as his campaign color, and it became pervasive among his supporters during the campaign and the post-election protests.^[63] Green is the lowest of the three bands on the flag of India. The green stands for fertility and prosperity. Earlier Indian flags had contained a similar green band representing Islam, the second-most predominant religion in India.^[64]

Vert tincture

Other countries use flags for reasons of heraldry, or to represent lush national vegetation. In heraldry, green is called vert (French for "green"). Fourteenth century documents describe vert as a symbol of "jolliness and youth, but also of beauty and shame" as well as of death.^{[citation needed]} Vert is used for the flags of Wales and Hungary, and is the basis for the Brazilian flag as well — here as a tribute to Positivist movement which Auguste Comte had connected to the colour both as it stood for life and as it seemed to him powerful as the color of Islam.^[65][66] Other countries using green in their flags use it to represent their country's lush vegetation, as in the flag of Jamaica,^[67] and hope in the future, as in the flags of Portugal and Nigeria.^[68]

Wikisource has original text related to this article: The Wearing of the Green

Green is a symbol of Ireland, which is often referred to as the "Emerald Isle". The color is particularly identified with the republican and nationalist traditions in modern times. It is used this way on the flag of the Republic of Ireland, in balance with white and the Protestant orange.^[69] Green is a strong trend in the Irish holiday St. Patrick’s Day.^[70]

Green has become the symbolic color of environmentalism, chosen for its association with nature, health, and growth. The Green Party is any of various political parties emphasizing ecology, grassroots democracy, nonviolence, and social justice. Green Parties, now active in over one hundred countries, are more broadly included in the green movement, and most are members of the Global Green Network.^[71] The association of green with advocates of the environment has extended to other circles as well, as is the case with Patriarch Bartholomew I of Constantinople, who is often referred to as the "Green Patriarch" because the new environmental focus which he brought about within the Ecumenical Patriarchate.^[72]

Religion[link]

A green-colored strip in the modern flag of Libya.

Green is considered the traditional color of Islam. Muhammad is reliably quoted in a hadith as saying that "water, greenery, and a beautiful face" were three universally good things.^[73] In the Qur'an, sura Al-Insan, believers in God in Paradise wear fine green silk.^[3][74] Also, Al-Khidr ("The Green One"), is a Qur’anic figure who met and traveled with Moses.^[75]

Roman Catholic and more traditional Protestant clergy wear green vestments at liturgical celebrations during Ordinary Time.^[76] In the Eastern Catholic Church, green is the color of Pentecost.^[77] Green is one of the Christmas colors as well, possibly dating back to pre-Christian times, when evergreens were worshiped for their ability to maintain their color through the winter season. Romans used green holly and evergreen as decorations for their winter solstice celebration called Saturnalia, which eventually evolved into a Christmas celebration.^[78] In Ireland and Scotland especially, green is used to represent Catholics, while orange is used to represent Protestantism. This is shown on the national flag of Ireland.

Metaphysics[link]

In the metaphysics of the "New Age Prophetess", Alice Bailey, in her system called the Seven Rays which classifies humans into seven different metaphysical psychological types, the "third ray" of "creative intelligence" is represented by the color green. People who have this metaphysical psychological type are said to be "on the Green Ray".^[79] In Hinduism, Green is used to symbolically represent the fourth, heart chakra (Anahata).^[80] Psychics who claim to be able to observe the aura with their third eye report that someone with a green aura is typically someone who is in an occupation related to health, such as a physician or nurse, as well as people who are lovers of nature and the outdoors.^[81]

See also[link]

• List of colors
• Variations of green

Notes and references[link]

External links[link]

Find more about Green on Wikipedia's sister projects:
	Definitions and translations from Wiktionary
	Images and media from Commons
	Learning resources from Wikiversity
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• Green All Over — slideshow by Life magazine

Colored pencils

Color or colour (see spelling differences) is the visual perceptual property corresponding in humans to the categories called red, green, blue, and others. Color derives from the spectrum of light (distribution of light power versus wavelength) interacting in the eye with the spectral sensitivities of the light receptors. Color categories and physical specifications of color are also associated with objects, materials, light sources, etc., based on their physical properties such as light absorption, reflection, or emission spectra. By defining a color space, colors can be identified numerically by their coordinates.

Because perception of color stems from the varying spectral sensitivity of different types of cone cells in the retina to different parts of the spectrum, colors may be defined and quantified by the degree to which they stimulate these cells. These physical or physiological quantifications of color, however, do not fully explain the psychophysical perception of color appearance.

The science of color is sometimes called chromatics, chromatography, colorimetry, or simply color science. It includes the perception of color by the human eye and brain, the origin of color in materials, color theory in art, and the physics of electromagnetic radiation in the visible range (that is, what we commonly refer to simply as light).

Physics[link]

Continuous optical spectrum rendered into the sRGB color space.

Electromagnetic radiation is characterized by its wavelength (or frequency) and its intensity. When the wavelength is within the visible spectrum (the range of wavelengths humans can perceive, approximately from 390 nm to 750 nm), it is known as "visible light".

Most light sources emit light at many different wavelengths; a source's spectrum is a distribution giving its intensity at each wavelength. Although the spectrum of light arriving at the eye from a given direction determines the color sensation in that direction, there are many more possible spectral combinations than color sensations. In fact, one may formally define a color as a class of spectra that give rise to the same color sensation, although such classes would vary widely among different species, and to a lesser extent among individuals within the same species. In each such class the members are called metamers of the color in question.

Spectral colors[link]

The familiar colors of the rainbow in the spectrum – named using the Latin word for appearance or apparition by Isaac Newton in 1671 – include all those colors that can be produced by visible light of a single wavelength only, the pure spectral or monochromatic colors. The table at right shows approximate frequencies (in terahertz) and wavelengths (in nanometers) for various pure spectral colors. The wavelengths are measured in air or vacuum (see refraction).

The color table should not be interpreted as a definitive list – the pure spectral colors form a continuous spectrum, and how it is divided into distinct colors linguistically is a matter of culture and historical contingency (although people everywhere have been shown to perceive colors in the same way^[2]). A common list identifies six main bands: red, orange, yellow, green, blue, and violet. Newton's conception included a seventh color, indigo, between blue and violet. Optical scientists Hardy and Perrin list indigo as between 446 and 464 nm wavelength.^[3]

The intensity of a spectral color, relative to the context in which it is viewed, may alter its perception considerably; for example, a low-intensity orange-yellow is brown, and a low-intensity yellow-green is olive-green.

For discussion of non-spectral colors, see below.

Color of objects[link]

The color of an object depends on both the physics of the object in its environment and the characteristics of the perceiving eye and brain. Physically, objects can be said to have the color of the light leaving their surfaces, which normally depends on the spectrum of the incident illumination and the reflectance properties of the surface, as well as potentially on the angles of illumination and viewing. Some objects not only reflect light, but also transmit light or emit light themselves (see below), which contribute to the color also. And a viewer's perception of the object's color depends not only on the spectrum of the light leaving its surface, but also on a host of contextual cues, so that the color tends to be perceived as relatively constant: that is, relatively independent of the lighting spectrum, viewing angle, etc. This effect is known as color constancy.

The upper disk and the lower disk have exactly the same objective color, and are in identical gray surroundings; based on context differences, humans perceive the squares as having different reflectances, and may interpret the colors as different color categories; see checker shadow illusion.

Some generalizations of the physics can be drawn, neglecting perceptual effects for now:

• Light arriving at an opaque surface is either reflected "specularly" (that is, in the manner of a mirror), scattered (that is, reflected with diffuse scattering), or absorbed – or some combination of these.
• Opaque objects that do not reflect specularly (which tend to have rough surfaces) have their color determined by which wavelengths of light they scatter more and which they scatter less (with the light that is not scattered being absorbed). If objects scatter all wavelengths, they appear white. If they absorb all wavelengths, they appear black.
• Opaque objects that specularly reflect light of different wavelengths with different efficiencies look like mirrors tinted with colors determined by those differences. An object that reflects some fraction of impinging light and absorbs the rest may look black but also be faintly reflective; examples are black objects coated with layers of enamel or lacquer.
• Objects that transmit light are either translucent (scattering the transmitted light) or transparent (not scattering the transmitted light). If they also absorb (or reflect) light of various wavelengths differentially, they appear tinted with a color determined by the nature of that absorption (or that reflectance).
• Objects may emit light that they generate themselves, rather than merely reflecting or transmitting light. They may do so because of their elevated temperature (they are then said to be incandescent), as a result of certain chemical reactions (a phenomenon called chemoluminescence), or for other reasons (see the articles Phosphorescence and List of light sources).
• Objects may absorb light and then as a consequence emit light that has different properties. They are then called fluorescent (if light is emitted only while light is absorbed) or phosphorescent (if light is emitted even after light ceases to be absorbed; this term is also sometimes loosely applied to light emitted because of chemical reactions).

For further treatment of the color of objects, see structural color, below.

To summarize, the color of an object is a complex result of its surface properties, its transmission properties, and its emission properties, all of which factors contribute to the mix of wavelengths in the light leaving the surface of the object. The perceived color is then further conditioned by the nature of the ambient illumination, and by the color properties of other objects nearby, via the effect known as color constancy and via other characteristics of the perceiving eye and brain.

Perception[link]

Normalized typical human cone cell responses (S, M, and L types) to monochromatic spectral stimuli

Development of theories of color vision[link]

Although Aristotle and other ancient scientists had already written on the nature of light and color vision, it was not until Newton that light was identified as the source of the color sensation. In 1810, Goethe published his comprehensive Theory of Colors in which he described the physiological effects of color.

In 1801 Thomas Young proposed his trichromatic theory, based on the observation that any color could be matched with a combination of three lights. This theory was later refined by James Clerk Maxwell and Hermann von Helmholtz. As Helmholtz puts it, "the principles of Newton's law of mixture were experimentally confirmed by Maxwell in 1856. Young's theory of color sensations, like so much else that this marvellous investigator achieved in advance of his time, remained unnoticed until Maxwell directed attention to it."^[4]

At the same time as Helmholtz, Ewald Hering developed the opponent process theory of color, noting that color blindness and afterimages typically come in opponent pairs (red-green, blue-orange, yellow-purple, and black-white). Ultimately these two theories were synthesized in 1957 by Hurvich and Jameson, who showed that retinal processing corresponds to the trichromatic theory, while processing at the level of the lateral geniculate nucleus corresponds to the opponent theory.^[5]

In 1931, an international group of experts known as the Commission internationale de l'éclairage (CIE) developed a mathematical color model, which mapped out the space of observable colors and assigned a set of three numbers to each.

Color in the eye[link]

This image (when viewed in full size, 1000 pixels wide) contains 1 million pixels, each of a different color. The human eye can distinguish about 10 million different colors.^[6]

The ability of the human eye to distinguish colors is based upon the varying sensitivity of different cells in the retina to light of different wavelengths. Humans being trichromatic, the retina contains three types of color receptor cells, or cones. One type, relatively distinct from the other two, is most responsive to light that we perceive as violet, with wavelengths around 420 nm; cones of this type are sometimes called short-wavelength cones, S cones, or blue cones. The other two types are closely related genetically and chemically. One of them, sometimes called long-wavelength cones, L cones, or red cones, is most sensitive to light we perceive as greenish yellow, with wavelengths around 564 nm; the other type, known as middle-wavelength cones, M cones, or green cones is most sensitive to light perceived as green, with wavelengths around 534 nm.

Light, no matter how complex its composition of wavelengths, is reduced to three color components by the eye. For each location in the visual field, the three types of cones yield three signals based on the extent to which each is stimulated. These amounts of stimulation are sometimes called tristimulus values.

The response curve as a function of wavelength for each type of cone is illustrated above. Because the curves overlap, some tristimulus values do not occur for any incoming light combination. For example, it is not possible to stimulate only the mid-wavelength (so-called "green") cones; the other cones will inevitably be stimulated to some degree at the same time. The set of all possible tristimulus values determines the human color space. It has been estimated that humans can distinguish roughly 10 million different colors.^[6]

The other type of light-sensitive cell in the eye, the rod, has a different response curve. In normal situations, when light is bright enough to strongly stimulate the cones, rods play virtually no role in vision at all.^[7] On the other hand, in dim light, the cones are understimulated leaving only the signal from the rods, resulting in a colorless response. (Furthermore, the rods are barely sensitive to light in the "red" range.) In certain conditions of intermediate illumination, the rod response and a weak cone response can together result in color discriminations not accounted for by cone responses alone. These effects, combined, are summarized also in the Kruithof curve, that describes the change of color perception and pleasingness of light as function of temperature and intensity.

Color in the brain[link]

The visual dorsal stream (green) and ventral stream (purple) are shown. The ventral stream is responsible for color perception.

While the mechanisms of color vision at the level of the retina are well-described in terms of tristimulus values (see above), color processing after that point is organized differently. A dominant theory of color vision proposes that color information is transmitted out of the eye by three opponent processes, or opponent channels, each constructed from the raw output of the cones: a red–green channel, a blue–yellow channel, and a black–white "luminance" channel. This theory has been supported by neurobiology, and accounts for the structure of our subjective color experience. Specifically, it explains why we cannot perceive a "reddish green" or "yellowish blue", and it predicts the color wheel: it is the collection of colors for which at least one of the two color channels measures a value at one of its extremes.

The exact nature of color perception beyond the processing already described, and indeed the status of color as a feature of the perceived world or rather as a feature of our perception of the world, is a matter of complex and continuing philosophical dispute (see qualia).

Nonstandard color perception[link]

Color deficiency[link]

If one or more types of a person's color-sensing cones are missing or less responsive than normal to incoming light, that person can distinguish fewer colors and is said to be color deficient or color blind (though this latter term can be misleading; almost all color deficient individuals can distinguish at least some colors). Some kinds of color deficiency are caused by anomalies in the number or nature of cones in the retina. Others (like central or cortical achromatopsia) are caused by neural anomalies in those parts of the brain where visual processing takes place.

Tetrachromacy[link]

While most humans are trichromatic (having three types of color receptors), many animals, known as tetrachromats, have four types. These include some species of spiders, most marsupials, birds, reptiles, and many species of fish. Other species are sensitive to only two axes of color or do not perceive color at all; these are called dichromats and monochromats respectively. A distinction is made between retinal tetrachromacy (having four pigments in cone cells in the retina, compared to three in trichromats) and functional tetrachromacy (having the ability to make enhanced color discriminations based on that retinal difference). As many as half of all women are retinal tetrachromats.^[8]:p.256 The phenomenon arises when an individual receives two slightly different copies of the gene for either the medium- or long-wavelength cones, which are carried on the x-chromosome. To have two different genes, a person must have two x-chromosomes, which is why the phenomenon only occurs in women.^[8] For some of these retinal tetrachromats, color discriminations are enhanced, making them functional tetrachromats.^[8]

Synesthesia[link]

In certain forms of synesthesia, perceiving letters and numbers (grapheme–color synesthesia) or hearing musical sounds (music–color synesthesia) will lead to the unusual additional experiences of seeing colors. Behavioral and functional neuroimaging experiments have demonstrated that these color experiences lead to changes in behavioral tasks and lead to increased activation of brain regions involved in color perception, thus demonstrating their reality, and similarity to real color percepts, albeit evoked through a non-standard route.

Afterimages[link]

An example of an Afterimage

After exposure to strong light in their sensitivity range, photoreceptors of a given type become desensitized. For a few seconds after the light ceases, they will continue to signal less strongly than they otherwise would. Colors observed during that period will appear to lack the color component detected by the desensitized photoreceptors. This effect is responsible for the phenomenon of afterimages, in which the eye may continue to see a bright figure after looking away from it, but in a complementary color.

Afterimage effects have also been utilized by artists, including Vincent van Gogh.

Color constancy[link]

There is an interesting phenomenon which occurs when an artist uses a limited color palette: the eye tends to compensate by seeing any gray or neutral color as the color which is missing from the color wheel. For example, in a limited palette consisting of red, yellow, black, and white, a mixture of yellow and black will appear as a variety of green, a mixture of red and black will appear as a variety of purple, and pure gray will appear bluish.^[9]

The trichromatic theory discussed above is strictly true when the visual system is in a fixed state of adaptation. In reality, the visual system is constantly adapting to changes in the environment and compares the various colors in a scene to reduce the effects of the illumination. If a scene is illuminated with one light, and then with another, as long as the difference between the light sources stays within a reasonable range, the colors in the scene appear relatively constant to us. This was studied by Edwin Land in the 1970s and led to his retinex theory of color constancy.

It should be noted, that both phenomena described above are readily explained and mathematical modeled with modern theories of chromatic adaptation and color appearance (e.g. CIECAM02, iCAM).^[10] There is no need to dismiss the trichromatic theory of vision, but rather it must be enhanced with an understanding of how the visual system adapts (adjusts) to changes in the viewing environment.

Color naming[link]

Colors vary in several different ways, including hue (shades of red, orange, yellow, green, blue, and violet), saturation, brightness, and gloss. Some color words are derived from the name of an object of that color, such as "orange" or "salmon", while others are abstract, like "red".

In the 1969 study Basic Color Terms: Their Universality and Evolution, Brent Berlin and Paul Kay describe a pattern in naming "basic" colors (like "red" but not "red-orange" or "dark red" or "blood red", which are "shades" of red). All languages that have two "basic" color names distinguish dark/cool colors from bright/warm colors. The next colors to be distinguished are usually red and then yellow or green. All languages with six "basic" colors include black, white, red, green, blue, and yellow. The pattern holds up to a set of twelve: black, gray, white, pink, red, orange, yellow, green, blue, purple, brown, and azure (distinct from blue in Russian and Italian, but not English).

Associations[link]

Individual colors have a variety of cultural associations such as national colors (in general described in individual color articles and color symbolism). The field of color psychology attempts to identify the effects of color on human emotion and activity. Chromotherapy is a form of alternative medicine attributed to various Eastern traditions. Colors have different associations in different countries and cultures.^[11]

Different colors have been demonstrated to have effects on cognition. For example, researchers at the University of Linz in Austria demonstrated that the color red significantly decreases cognitive functioning in men.^[12]

Spectral colors and color reproduction[link]

The CIE 1931 color space chromaticity diagram. The outer curved boundary is the spectral (or monochromatic) locus, with wavelengths shown in nanometers. The colors depicted depend on the color space of the device on which you are viewing the image, and therefore may not be a strictly accurate representation of the color at a particular position, and especially not for monochromatic colors.

Most light sources are mixtures of various wavelengths of light. Many such sources can still effectively produce a spectral color, as the eye cannot distinguish them from single-wavelength sources. For example, most computer displays reproduce the spectral color orange as a combination of red and green light; it appears orange because the red and green are mixed in the right proportions to allow the eye's cones to respond the way they do to the spectral color orange.

A useful concept in understanding the perceived color of a non-monochromatic light source is the dominant wavelength, which identifies the single wavelength of light that produces a sensation most similar to the light source. Dominant wavelength is roughly akin to hue.

There are many color perceptions that by definition cannot be pure spectral colors due to desaturation or because they are purples (mixtures of red and violet light, from opposite ends of the spectrum). Some examples of necessarily non-spectral colors are the achromatic colors (black, gray, and white) and colors such as pink, tan, and magenta.

Two different light spectra that have the same effect on the three color receptors in the human eye will be perceived as the same color. They are metamers of that color. This is exemplified by the white light emitted by fluorescent lamps, which typically has a spectrum of a few narrow bands, while daylight has a continuous spectrum. The human eye cannot tell the difference between such light spectra just by looking into the light source, although reflected colors from objects can look different. (This is often exploited; for example, to make fruit or tomatoes look more intensely red.)

Similarly, most human color perceptions can be generated by a mixture of three colors called primaries. This is used to reproduce color scenes in photography, printing, television, and other media. There are a number of methods or color spaces for specifying a color in terms of three particular primary colors. Each method has its advantages and disadvantages depending on the particular application.

No mixture of colors, however, can produce a fully pure color perceived as completely identical to a spectral color, although one can get very close for the longer wavelengths, where the chromaticity diagram above has a nearly straight edge. For example, mixing green light (530 nm) and blue light (460 nm) produces cyan light that is slightly desaturated, because response of the red color receptor would be greater to the green and blue light in the mixture than it would be to a pure cyan light at 485 nm that has the same intensity as the mixture of blue and green.

Because of this, and because the primaries in color printing systems generally are not pure themselves, the colors reproduced are never perfectly saturated colors, and so spectral colors cannot be matched exactly. However, natural scenes rarely contain fully saturated colors, thus such scenes can usually be approximated well by these systems. The range of colors that can be reproduced with a given color reproduction system is called the gamut. The CIE chromaticity diagram can be used to describe the gamut.

Another problem with color reproduction systems is connected with the acquisition devices, like cameras or scanners. The characteristics of the color sensors in the devices are often very far from the characteristics of the receptors in the human eye. In effect, acquisition of colors that have some special, often very "jagged", spectra caused for example by unusual lighting of the photographed scene can be relatively poor.

Species that have color receptors different from humans – such as bird species, which may have four receptors – can make color discriminations that humans cannot. A color reproduction system "tuned" to a human with normal color vision may give very inaccurate results for the other observers, human or non-human.

The different color response of different devices can be problematic if not properly managed. For color information stored and transferred in digital form, color management techniques, such as those based on ICC profiles, can help to avoid distortions of the reproduced colors. Color management does not circumvent the gamut limitations of particular output devices, but can assist in finding good mapping of input colors into the gamut that can be reproduced.

Pigments and reflective media[link]

Pigments are chemicals that selectively absorb and reflect different spectra of light. When a surface is painted with a pigment, light hitting the surface is reflected, minus some wavelengths. This subtraction of wavelengths produces the appearance of different colors. Most paints are a blend of several chemical pigments, intended to produce a reflection of a given color.

Pigment manufacturers assume the source light will be white, or of roughly equal intensity across the spectrum. If the light is not a pure white source (as in the case of nearly all forms of artificial lighting), the resulting spectrum will appear a slightly different color. Red paint, viewed under blue light, may appear black. Red paint is red because it reflects only the red components of the spectrum. Blue light, containing none of these, will create no reflection from red paint, creating the appearance of black.

Structural color[link]

Structural colors are colors caused by interference effects rather than by pigments. Color effects are produced when a material is scored with fine parallel lines, formed of one or more parallel thin layers, or otherwise composed of microstructures on the scale of the color's wavelength. If the microstructures are spaced randomly, light of shorter wavelengths will be scattered preferentially to produce Tyndall effect colors: the blue of the sky (Rayleigh scattering, caused by structures much smaller than the wavelength of light, in this case air molecules), the luster of opals, and the blue of human irises. If the microstructures are aligned in arrays, for example the array of pits in a CD, they behave as a diffraction grating: the grating reflects different wavelengths in different directions due to interference phenomena, separating mixed "white" light into light of different wavelengths. If the structure is one or more thin layers then it will reflect some wavelengths and transmit others, depending on the layers' thickness.

Structural color is studied in the field of thin-film optics. A layman's term that describes particularly the most ordered or the most changeable structural colors is iridescence. Structural color is responsible for the blues and greens of the feathers of many birds (the blue jay, for example), as well as certain butterfly wings and beetle shells. Variations in the pattern's spacing often give rise to an iridescent effect, as seen in peacock feathers, soap bubbles, films of oil, and mother of pearl, because the reflected color depends upon the viewing angle. Numerous scientists have carried out research in butterfly wings and beetle shells, including Isaac Newton and Robert Hooke. Since 1942, electron micrography has been used, advancing the development of products that exploit structural color, such as "photonic" cosmetics.^[13]

Additional terms[link]

• Colorfulness, chroma, purity, or saturation: how "intense" or "concentrated" a color is. Technical definitions distinguish between colorfulness, chroma, and saturation as distinct perceptual attributes and include purity as a physical quantity. These terms, and others related to light and color are internationally agreed upon and published in the CIE Lighting Vocabulary.^[14] More readily available texts on colorimetry also define and explain these terms.^[10][15]
• Dichromatism: a phenomenon where the hue is dependent on concentration and/or thickness of the absorbing substance.
• Hue: the color's direction from white, for example in a color wheel or chromaticity diagram.
• Shade: a color made darker by adding black.
• Tint: a color made lighter by adding white.
• Value, brightness, lightness, or luminosity: how light or dark a color is.

See also[link]

Find more about Color on Wikipedia's sister projects:
	Definitions and translations from Wiktionary
	Images and media from Commons

• Chromophore
• CIECAM02
• Color analysis
• Color mapping
• Complementary color
• Impossible color
• International Color Consortium
• International Commission on Illumination
• List of colors (compact version)
• Neutral color
• Pearlescent coating including Metal effect pigments
• Primary, secondary and tertiary colors
• Rainbow

References[link]

External links and sources[link]

• Bibliography Database on Color Theory, Buenos Aires University
• Color entry by Barry Maund in the Stanford Encyclopedia of Philosophy
• Why Should Engineers and Scientists Be Worried About Color?
• Robert Ridgway's A Nomenclature of Colors (1886) and Color Standards and Color Nomenclature (1912) - text-searchable digital facsimiles at Linda Hall Library
• Albert Henry Munsell's A Color Notation, (1907) at Project Gutenberg
• AIC, International Colour Association

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