Editing RGB color model (section)

==Physical principles for the choice of red, green, and blue==
[[File:CIExy1931 sRGB gamut D65.png|right|thumb|A set of primary colors, such as the [[sRGB]] primaries, define a [[color triangle]]; only colors within this triangle can be reproduced by mixing the primary colors. Colors outside the color triangle are therefore shown here as gray. The primaries and the [[CIE Standard Illuminant D65|D65]] [[white point]] of sRGB are shown. The background figure is the [[CIE 1931 color space|CIE xy chromaticity diagram]].]]
The choice of primary colors is related to the physiology of the [[human eye]]; good primaries are stimuli that maximize the difference between the responses of the [[cone cell]]s of the human retina to light of different [[wavelength]]s, and that thereby make a large [[color triangle]].<ref name=RWGHunt>{{cite book
 | author = R. W. G. Hunt
 | year = 2004
 | title = The Reproduction of Colour
 | edition = 6th
 | publisher = Wiley–IS&T Series in Imaging Science and Technology
 | location = Chichester UK
 | isbn = 0-470-02425-9
 | url-access = registration
 | url = https://archive.org/details/reproductionofco0000hunt
 }}</ref>

The normal three kinds of light-sensitive [[photoreceptor cell]]s in the human eye (cone cells) respond most to yellow (long wavelength or L), green (medium or M), and violet (short or S) light (peak wavelengths near 570&nbsp;nm, 540&nbsp;nm and 440&nbsp;nm, respectively<ref name=RWGHunt/>). The difference in the signals received from the three kinds allows the brain to differentiate a wide [[gamut]] of different colors, while being most sensitive (overall) to yellowish-green light and to differences between hues in the green-to-orange region.

As an example, suppose that light in the [[Orange (colour)|orange]] range of wavelengths (approximately 577&nbsp;nm to 597&nbsp;nm) enters the eye and strikes the retina. Light of these wavelengths would activate both the medium and long wavelength cones of the retina, but not equally—the long-wavelength cells will respond more. The difference in the response can be detected by the brain, and this difference is the basis of our perception of orange. Thus, the orange appearance of an object results from light from the object entering our eye and stimulating the different cones simultaneously but to different degrees.

Use of the three primary colors is not sufficient to reproduce ''all'' colors; only colors within the color triangle defined by the chromaticities of the primaries can be reproduced by additive mixing of non-negative amounts of those colors of light.<ref name=RWGHunt/>{{Page needed|date=February 2015}}
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