The CIE XYZ (1931) system is at the root of all colorimetry. It is defined such that all visible colours can be defined using only positive values, and, the Y value is luminance. Consequently, the colours of the XYZ primaries themselves are not visible. The chromaticity diagram is highly non-linear, in that a vector of unit magnitude representing the difference between two chromaticities is not uniformly visible. A colour defined in this system is referred to as Yxy. A third coordinate, z, can also be defined but is redundant since x+y+z=1 for all colours.
x = X / (X + Y + Z) y = Y / (X + Y + Z)