Color effect - Sunlight shining through stained glass onto carpet (Nasir ol Molk Mosque located in Shiraz, Iran)
Colors can look differently depending on their surrounding colours and shapes. The two small squares have exactly the same color, but the right one looks slightly darker.

Color (American English) or colour (Commonwealth English) is the visual perceptual property corresponding in humans to the categories called red, blue, yellow, etc. 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 colour are additionally associated with objects or materials based on their physical properties such as light absorption, reflection, or emission spectra. By defining a color space colours can be identified numerically by their coordinates.

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

The science of colour is at times called chromatics, colorimetry, or simply color science. It includes the perception of colour by the human eye and brain, the origin of colour in materials, color theory in art, and the physics of electromagnetic radiation in the visible range (that is, what's commonly referred to simply as light).

Physics of color

Continuous optical spectrum rendered into the sRGB colour space.
The colours of the visible light spectrum
colorWavelength
interval
Frequency
interval
Red~ 700–635 nm~ 430–480 THz
Orange~ 635–590 nm~ 480–510 THz
Yellow~ 590–560 nm~ 510–540 THz
Green~ 560–520 nm~ 540–580 THz
Cyan~ 520–490 nm~ 580–610 THz
Blue~ 490–450 nm~ 610–670 THz
Violet~ 450–400 nm~ 670–750 THz
Color, wavelength, frequency and energy of light
Color

(nm)

(THz)

(μm−1)

(eV)

(kJ mol−1)

Infrared>1000<300<1.00<1.24<120
Red7004281.431.77171
Orange6204841.612.00193
Yellow5805171.722.14206
Green5305661.892.34226
Blue4706382.132.64254
Violet4207142.382.95285
Near ultraviolet30010003.334.15400
Far ultraviolet<200>1500>5.00>6.20>598


Electromagnetic radiation is characterised 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 700 nm), it is known as "visible light".

Most light sources emit light at a large number of 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 colour sensation in that direction, there are a large number of more possible spectral combinations than colour sensations. In fact, one might formally define a colour as a class of spectra that give rise to the same colour 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 colour in question.

Spectral colors

The familiar colours of the rainbow in the spectrum – named using the Latin word for appearance or apparition by Isaac Newton in 1671 – include all those colours 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 listed are as measured in air or vacuum (see refractive index).

The colour table shouldn't be interpreted as a definitive list – the pure spectral colours form a continuous spectrum, and how it is divided into distinct colours linguistically is a matter of culture and historical contingency (although people everywhere have been shown to perceive colours in the same way). 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. It is possible that what Newton referred to as blue is nearer to what today is known as cyan, and that indigo was simply the dark blue of the indigo dye that was being imported at the time.

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

Color of objects

The colour 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 colour 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 additionally transmit light or emit light themselves, which additionally contribute to the color. A viewer's perception of the object's colour depends not only on the spectrum of the light leaving its surface, but additionally on a host of contextual cues, so that colour differences between objects can be discerned mostly independent of the lighting spectrum, viewing angle, etc. This effect is known as color constancy.

The upper disc and the lower disc have exactly the same objective color, and are in identical grey surroundings; based on context differences, humans perceive the squares as having different reflectances, and might interpret the colours as different colour categories; see checker shadow illusion.

Some generalisations 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 a few combination of these.
  • Opaque objects that don't reflect specularly (which tend to have rough surfaces) have their colour determined by which wavelengths of light they scatter strongly (with the light that isn't scattered being absorbed). If objects scatter all wavelengths with roughly equal strength, 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 colours determined by those differences. An object that reflects a few fraction of impinging light and absorbs the rest might look black but additionally 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 additionally absorb (or reflect) light of various wavelengths differentially, they appear tinted with a colour determined by the nature of that absorption (or that reflectance).
  • Objects might emit light that they generate from having excited electrons, rather than merely reflecting or transmitting light. The electrons might be excited due to elevated temperature (incandescence), as a result of chemical reactions (chemoluminescence), after absorbing light of additional frequencies ("fluorescence" or "phosphorescence") or from electrical contacts as in light emitting diodes, or additional light sources.

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

Perception

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.

Development of theories of colour vision

Although Aristotle and additional ancient scientists had already written on the nature of light and color vision, it wasn't until Newton that light was identified as the source of the colour sensation. In 1810, Goethe published his comprehensive Theory of Colors in which he ascribed physiological effects to colour that are now understood as psychological.

In 1801 Thomas Young proposed his trichromatic theory, based on the observation that any colour can 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 colour sensations, like so much else that this marvellous investigator achieved in advance of his time, remained unnoticed until Maxwell directed attention to it."

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-violet, and black-white). Ultimately these two theories were synthesised 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.

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

Color in the eye

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

The ability of the human eye to distinguish colours 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 colour receptor cells, or cones. One type, relatively distinct from the additional two, is most responsive to light that's perceived as blue or blue-violet, with wavelengths around 450 nm; cones of this type are at times called short-wavelength cones, S cones, or blue cones. The additional two types are closely related genetically and chemically: middle-wavelength cones, M cones, or green cones are most sensitive to light perceived as green, with wavelengths around 540 nm, while the long-wavelength cones, L cones, or red cones, are most sensitive to light is perceived as greenish yellow, with wavelengths around 570  nm.

Light, no matter how complex its composition of wavelengths, is reduced to three colour 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 at times called tristimulus values.

The response curve as a function of wavelength varies for each type of cone. Because the curves overlap, a few tristimulus values don't occur for any incoming light combination. For example, it isn't possible to stimulate only the mid-wavelength (so-called "green") cones; the additional cones will inevitably be stimulated to a few 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.

The additional 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. On the additional 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 colour discriminations not accounted for by cone responses alone. These effects, combined, are summarised additionally in the Kruithof curve, that describes the change of colour perception and pleasingness of light as function of temperature and intensity.

Color in the brain

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

While the mechanisms of colour vision at the level of the retina are well-described in terms of tristimulus values, colour processing after that point is organised differently. A dominant theory of colour vision proposes that colour 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 colour experience. Specifically, it explains why humans can't perceive a "reddish green" or "yellowish blue", and it predicts the color wheel: it is the collection of colours for which at least one of the two colour channels measures a value at one of its extremes.

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

Nonstandard colour perception

Color deficiency

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 colours and is said to be color deficient or color blind (though this latter term can be misleading; almost all colour deficient individuals can distinguish at least a few colors). Some kinds of colour 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

While most humans are trichromatic (having three types of colour receptors), a large number of animals, known as tetrachromats, have four types. These include a few species of spiders, most marsupials, birds, reptiles, and a large number of species of fish. Other species are sensitive to only two axes of colour or don't perceive colour 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 colour discriminations based on that retinal difference). As a large number of as half of all women are retinal tetrachromats.: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. For a few of these retinal tetrachromats, colour discriminations are enhanced, making them functional tetrachromats.

Synesthesia

In certain forms of synesthesia/ideasthesia, 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 colour experiences lead to changes in behavioural tasks and lead to increased activation of brain regions involved in colour perception, thus demonstrating their reality, and similarity to real colour percepts, albeit evoked through a non-standard route.

Afterimages

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'll continue to signal less strongly than they otherwise would. Colors observed throughout that period will appear to lack the colour component detected by the desensitised photoreceptors. This effect is responsible for the phenomenon of afterimages, in which the eye might continue to see a bright figure after looking away from it, but in a complementary color.

Afterimage effects have additionally been utilised by artists, including Vincent van Gogh.

Color constancy

When an artist uses a limited color palette, the eye tends to compensate by seeing any grey or neutral colour as the colour which is missing from the colour 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 grey will appear bluish.

The trichromatic theory 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 colours 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 colours 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 are readily explained and mathematically modelled with modern theories of chromatic adaptation and colour appearance (e.g. CIECAM02, iCAM). There is no need to dismiss the trichromatic theory of vision, but rather it can be enhanced with an understanding of how the visual system adapts to changes in the viewing environment.

Color naming

Colors vary in several different ways, including hue (shades of red, orange, yellow, green, blue, and violet), saturation, brightness, and gloss. Some colour 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" colours (like "red" but not "red-orange" or "dark red" or "blood red", which are "shades" of red). All languages that have two "basic" colour names distinguish dark/cool colours from bright/warm colors. The next colours to be distinguished are usually red and then yellow or green. All languages with six "basic" colours 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

Individual colours have a variety of cultural associations such as national colors (in general described in individual colour articles and color symbolism). The field of color psychology attempts to identify the effects of colour 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.

Different colours have been demonstrated to have effects on cognition. For example, researchers at the University of Linz in Austria demonstrated that the colour red significantly decreases cognitive functioning in men.

Spectral colours and colour reproduction

The CIE 1931 colour space chromaticity diagram. The outer curved boundary is the spectral (or monochromatic) locus, with wavelengths shown in nanometers. The colours depicted depend on the color space of the device on which you're viewing the image, and therefore might not be a strictly accurate representation of the colour 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 can't distinguish them from single-wavelength sources. For example, most computer displays reproduce the spectral colour 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 colour orange.

A useful concept in understanding the perceived colour 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 a large number of colour perceptions that by definition can't be pure spectral colours due to desaturation or because they're purples (mixtures of red and violet light, from opposite ends of the spectrum). Some examples of necessarily non-spectral colours are the achromatic colours (black, gray, and white) and colours such as pink, tan, and magenta.

Two different light spectra that have the same effect on the three colour 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 can't tell the difference between such light spectra just by looking into the light source, although reflected colours from objects can look different. (This is often exploited; for example, to make fruit or tomatoes look more intensely red.)

Similarly, most human colour perceptions can be generated by a mixture of three colours called primaries. This is used to reproduce colour scenes in photography, printing, television, and additional media. There are a number of methods or color spaces for specifying a colour 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 response truly identical to that of a spectral color, although one can get close, especially for the longer wavelengths, where the CIE 1931 colour space chromaticity diagram has a nearly straight edge. For example, mixing green light (530 nm) and blue light (460 nm) produces cyan light that's slightly desaturated, because response of the red colour 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 aren't pure themselves, the colours reproduced are never perfectly saturated spectral colors, and so spectral colours can't be matched exactly. Notwithstanding natural scenes rarely contain fully saturated colors, thus such scenes can usually be approximated well by these systems. The range of colours that can be reproduced with a given colour reproduction system is called the gamut. The CIE chromaticity diagram can be used to describe the gamut.

Another problem with colour reproduction systems is connected with the acquisition devices, like cameras or scanners. The characteristics of the colour sensors in the devices are often quite far from the characteristics of the receptors in the human eye. In effect, acquisition of colours can be relatively poor if they have special, often quite "jagged", spectra caused for example by unusual lighting of the photographed scene. A colour reproduction system "tuned" to a human with normal colour vision might give quite inaccurate results for additional observers.

The different colour response of different devices can be problematic if not properly managed. For colour 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 doesn't circumvent the gamut limitations of particular output devices, but can assist in finding good mapping of input colours into the gamut that can be reproduced.

Additive coloring

Additive colour mixing: adding red to green yields yellow; adding all three primary colours together yields white.

Additive color is light created by mixing together light of two or more different colors. Red, green, and blue are the additive primary colors normally used in additive colour systems such as projectors and computer terminals.

Subtractive coloring

Subtractive colour mixing: subtracting yellow from magenta yields red; subtracting all three primary colours together yields black

Subtractive coloring uses dyes, inks, pigments, or philtres to absorb a few wavelengths of light and not others. The colour that a surface displays comes from the parts of the visible spectrum that aren't absorbed and therefore remain visible. Without pigments or dye, fabric fibers, paint base and paper are usually made of particles that scatter white light (all colors) well in all directions. When a pigment or ink is added, wavelengths are absorbed or "subtracted" from white light, so light of another colour reaches the eye.

If the light isn't a pure white source (the case of nearly all forms of artificial lighting), the resulting spectrum will appear a slightly different color. Red paint, viewed under blue light, might appear black. Red paint is red because it scatters only the red components of the spectrum. If red paint is illuminated by blue light, it will be absorbed by the red paint, creating the appearance of a black object.

Structural color

Structural colours are colours 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 lustre 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 a few wavelengths and transmit others, depending on the layers' thickness.

Structural colour is studied in the field of thin-film optics. A layman's term that describes particularly the most ordered or the most changeable structural colours is iridescence. Structural colour is responsible for the blues and greens of the feathers of a large number of 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 colour 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.

Mentions of colour in social media

According to Pantone, the top three colours in social media for 2012 were red (186 million mentions; accredited to Taylor Swift's Red album, NASA's landing on Mars, and red carpet coverage), blue (125 million mentions; accredited to the United States presidential election, 2012, Mars rover Curiosity finding blue rocks, and blue sports teams), and Green (102 million mentions; accredited to "environmental friendliness", Green Bay Packers, and green eyed girls).

Additional terms

  • Color wheel: an illustrative organisation of colour hues in a circle that shows relationships.
  • Colorfulness, chroma, purity, or saturation: how "intense" or "concentrated" a colour 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 colour are internationally agreed upon and published in the CIE Lighting Vocabulary. More readily available texts on colorimetry additionally define and explain these terms.
  • 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 colour made darker by adding black.
  • Tint: a colour made lighter by adding white.
  • Value, brightness, lightness, or luminosity: how light or dark a colour is.