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Primary color

A primary color is one of a small set (usually three) colors from which other colors within the gamut[?] of a given color space can be made by mixing. If the color space is considered as a vector space, the primary colors can be regarded as a set of basis vectors for that space.

Additive primaries

Media that use emitted light and therefore additive color mixing (such as television) use the additive primaries red, green, and blue. Because of the response curves of the three different color receptors in the human eye, these colors are optimal in the sense that the largest range of colors visible by humans can be generated by mixing light of those colors. Adding together equal proportions of the additive primaries results in greyish colors and eventually white.

Note that primary colors are not a physical but rather a biological concept, based on the physiological response of the human eye to light. The peak responsitivites of the human eye's cone vision[?] color receptors do not occur exactly at the red, green and blue frequencies; rather, red, green and blue are chosen because they provide a wide gamut, making it possible to almost independently stimulate the three color receptors (whose response curves overlap).

In low light levels, a second set of light detectors called "rods" become important, leading to color shifts in human vision (see Kruithof curve). However, this does not invalidate the tri-color model of color perception.

To generate optimal color ranges for species other than humans, other additive primary colors would have to be used. For species with four different color receptors, such as many birds, one would use four primary colors.

Subtractive primaries

Media that use reflected light and therefore subtractive color mixing (like ink on paper) use the subtractive primaries yellow, cyan, (often called "blue", though this is a different hue from the usual additive blue primary), and magenta (likewise sometimes called "red"). With these subtractive primaries, the largest range of colors visible by humans can be generated. This is because yellow light is an equal mixture of red and green light (and thus a yellow surface absorbs blue light), cyan light is an equal mixture of green and blue light (and thus a cyan surface absorbs red light), and magenta is an equal mixture of red and blue light (and thus a magenta surface absorbs green light). Mixing together equal amounts of paints of the subtractive primaries results in greyish colors and eventually a muddy black.

 
Additive color mixing  Subtractive color mixing

Because the "black" generated by mixing the subtractive primaries is not as black as that of a genuine black ink (one that absorbs throughout the visible spectrum), four-color printing uses a fourth black ink in addition to yellow, magenta and cyan. The color space generated is the so-called CMYK color space (standing for "Cyan, Magenta, Yellow, and Key").

For a more detailed and extensive treatment of color, see color.

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