A set of primary colors is a small, arbitrary set of pigmented physical media, lights or purely abstract elements of a mathematical colorspace model. Distinct colors from a larger gamut can be specified in terms of a mixture of primary colors which facilitates technological applications such as painting, electronic displays and printing. Any small set of pigments or lights are "imperfect" physical primary colors in that they cannot be mixed to yield all possible colors that can be perceived by the human color vision system. The abstract (or "imaginary") primaries X, Y and Z of the CIEXYZ colorspace can be mathematically summed to specify essentially all colors that can be perceived but these primaries cannot be physically realized due to the underlying structure and overlapping spectral sensitivities of each of the human cone photoreceptors. The precise set of primary colors that are used in a specific color application depend on gamut requirements as well as application-specific constraints such as cost, power consumption, lightfastness, mixing behavior etc.
In an additive set of colors, as in coincident projected lights or in electronic visual displays, the primary colors normally used are red, green and blue (but the precise visible light spectra for each color can vary significantly). In a subtractive set of colors, as in mixing of pigments or dyes for printing, the colors magenta, yellow and cyan are normally used. See RGB color model, and CMYK color model for more on these popular sets of primary colors.
Primary colors are not a fundamental property of light but are related to the color vision system in animals. The human eye normally contains only three types of color photoreceptors (L, M and S) that are associated with specialized cone cells. Each photoreceptor responds to different ranges of the visible electromagnetic spectrum and there is no single wavelength that stimulates only one photoreceptor type. Humans and other species with three such types of color photoreceptor are known as trichromats. In spite of color being a complex psychophysical response to electromagnetic radiation, controlled color matching experiments (e.g., CIE 1931) have essentially mapped all possible colors the eye can see in terms of the response of each of the three color photoreceptors, which correspond to the three dimensions of CIEXYZ. Color appearance models like CIECAM02 describe color more generally in six dimensions and can be used to predict how colors appear in different viewing conditions.
Most placental mammals other than primates have only two types of color photoreceptor and are therefore dichromats while birds and marsupials are tetrachromats with four color photoreceptor types. There is no currently peer reviewed scholarly work that has confirmed the existence of a functional human tetrachromat though they are suspected to exist. It may seem that the primary colors of an animal's vision system corresponds to the number of color photoreceptor types but the mere presence of "extra" photoreceptor types does not directly imply that they are being used functionally. Demonstrating improved spectral discrimination in any animal can be difficult since complex sets of neurons affect color perception in ways that are generally difficult to interrogate.
Before the nature of colorimetry and visual physiology were well understood a number of color models assigned primary colors to different hues (e.g. the RYB model). Scientists such as Thomas Young, James Clerk Maxwell and Hermann von Helmholtz expressed various opinions about what should be the three primary colors to describe the three primary color sensations of the eye. Young originally proposed red, green and violet, and Maxwell changed violet to blue; Helmholtz proposed "a slightly purplish red, a vegetation-green, slightly yellowish, and an ultramarine-blue. In modern understanding, human cone cells do not correspond precisely to a specific set of primary colors, as each cone type responds to a relatively broad range of wavelengths.
Limited palettes in visual art
There are hundreds of commercially available pigments for visual artists to use and mix (in various media such as oil, watercolor, acrylic and pastel). A common approach is to use just a limited palette of pigments (often between four and eight) that can be physically mixed to any color that the artist desires in the final work. There are no specific set of pigments that are primary colors, the choice of pigment depends entirely on the artist's subjective preference of subject and style of art as well as material considerations like lightfastness and mixing heuristics. Contemporary classical realists have often advocated that a limited palette of white, red, yellow and black pigment (often described as the "Zorn palette") is sufficient for compelling work.
RGB for electronic displays
Media that combine emitted lights to create the sensation of a range of colors are using the additive color system. The primary colors used in most electronic displays are typically saturated red, green and blue light.
The exact colors chosen for the primaries are a technological compromise between the available phosphors (including considerations such as cost and power usage) and the need for large color triangle to allow a large gamut of colors. The ITU-R BT.709-5/sRGB primaries are typical. Additive mixing of red and green light produces shades of yellow, orange, or brown. Mixing green and blue produces shades of cyan, and mixing red and blue produces shades of purple, including magenta. Mixing nominally equal proportions of the additive primaries results in shades of grey or white; the color space that is generated is called an RGB color space. The experiments used to derive the CIE 1931 colorspace used monochromatic primary colored lights with the (arbitrary) wavelengths of 435.8 nm (violet), 546.1 nm (green) and 700 nm (red) due to the convenience they afforded to the experimental work.
Some recent TV and computer displays are starting to include yellow as a fourth primary color, often in a four-point square pixel area, so as to achieve brighter pure yellows and a larger color gamut. Even the four-primary technology does not yet reach the range of colors that the human eye can see from light reflected by illuminated surfaces (as defined by the sample-based estimate called the Pointer Gamut), with 4-primary LED prototypes providing typically about 87% and 5-primary prototypes about 95%. Several firms, including Samsung and Mitsubishi, have demonstrated LED displays with five or six "primaries", or color LED point light sources per pixel. A recent academic literature review claims a gamut of 99% can be achieved with 5-primary LED technology. While technology for achieving a wider gamut appears to be within reach, other issues remain; for example, affordability, dynamic range, and brilliance. In addition, there exists hardly any source material recorded in this wider gamut, nor is it currently possible to recover this information from existing visual media. Regardless, industry is still exploring a wide variety of "primary" active light sources (per pixel) with the goal of matching the capability of human color perception within a broadly affordable price. One example of a potentially affordable but yet unproven active light hybrid places an LED screen over a plasma light screen, each with different "primaries". Because both LED and plasma technologies are many decades old (plasma pixels going back to the 1960s), both have become so affordable that they could be combined.
CMYK color model or four-color printing
In the printing industry, the subtractive primaries cyan, magenta and yellow are applied together in varying amounts for useful gamuts. An additional key ink (shorthand for the key printing plate that impressed the artistic detail of an image, usually in black ink.) is also usually used since it is difficult to mix a dark enough black ink using the other three inks as well as other practical considerations such as cost and ink bleed. Before the color names cyan and magenta were in common use, these primaries were often known as blue-green and purple or in some pop art circles as blue and red, respectively, and their exact color has changed over time with access to new pigments and technologies.
The opponent process is a color theory that states that the human visual system interprets information about color by processing signals from cones and rods in an antagonistic manner. The three types of cones have some overlap in the wavelengths of light to which they respond, so it is more efficient for the visual system to record differences between the responses of cones, rather than each type of cone's individual response. The opponent color theory suggests that there are three opponent channels: red versus green, blue versus yellow and black versus white. Responses to one color of an opponent channel are antagonistic to those of the other color. The theory states that the particular colors considered by an observer to be uniquely representative of the concepts red, yellow, green, blue, white and black might be called "psychological primary colors", because any other color could be described in terms of some combination of these.
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