<P> The precise set of primary colors to be used in a specific color application depends on gamut requirements as well as on application - specific constraints such as cost, power consumption, lightfastness, mixing behavior, etc . </P> <P> Abstract colorspaces include the additive CIE XYZ, in which all visible colors can be represented mathematically by combinations of its X, Y, and Z primaries, but each primary itself does not directly correspond to a physically - possible light source . </P> <P> The response of the eye to a "mix" of primary colors is predicted by different models for different applications . </P> <P> In additive color mixing, the total light present is simply the sum of individual light sources . For example, coincident blue and red spotlights on a surface will make a purple, brighter than either of the spotlights alone . Additive mixing of coincident spot lights was applied in the experiments used to derive the CIE 1931 colorspace . The original monochromatic primaries of the (arbitrary) wavelengths of 435.8 nm (violet), 546.1 nm (green) and 700 nm (red) were used in this application due to the convenience they afforded to the experimental work . </P>

Which one of these is not a primary color