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Primary color

A primary color is a color that cannot be created by mixing other colors in the gamut of a given color space. Primary colors may themselves be mixed to produce most of the colors in a given color space. Often, the colors red, yellow, and blue are considered to be primary pigments. 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.

Biological basis

Primary colors are not a physical but rather a biological concept, based on the physiological response of the human eye to light. The human eye contains receptors called cones which normally respond to red, green, and blue light. Although the peak responsitivites of the cones do not occur exactly at the red, green and blue frequencies, those three colors are chosen as primary because they provide a wide gamut, making it possible to almost independently stimulate the three color receptors. 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.

Additive primaries

Additive color mixing

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. Red and green, when mixed, produce shades of yellow or orange. Mixing green and blue produces shades of cyan, and mixing red and blue produces shades of purple and magenta. Mixing equal proportions of the additive primaries results in shades of grey; if all three colors are mixed at "full-strength", the result is white.

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"). The subtractive color model works best when the surface (or paper) is white, or close to it.

Subtractive color mixing

Mixing yellow and cyan produces shades of green; mixing yellow with magenta produces shades of red, and mixing magenta with cyan produces shades of blue. In theory, mixing equal amounts of all three pigments should produce shades of grey, resulting in black when all three are mixed at "full-strength", but in practice they tend to produce muddy brown colors. For this reason, a fourth "primary" pigment, black, is often used in addition to the cyan, magenta, and yellow colors. The color space generated is the so-called CMYK color space (standing for "Cyan, Magenta, Yellow, and Key").

In practice, mixing actual medium such as paint tends to be less precise. Brighter, or more specific colors can be created using natural pigments instead of mixing, and natural properties of pigments can interfere with the mixing. For example, mixing black and yellow in acrylic creates green - something which would not happen if the mixing process were perfectly subtractive. Also, in the subtractive model, adding white to any color should not change the color at all (since white does not subtract any color) - yet again, in practice, mixing white with other pigments does alter their coloration.

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

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