The amounts of red, green, and blue needed to form any particular color are called the tristimulus values and are denoted X, Y, and Z, respectively. A color is then specified by its trichromatic coefficients, defined as

$ x=\frac{X}{X+Y+Z} $ and $ y=\frac{Y}{X+Y+Z} $ and $ z=\frac{Z}{X+Y+Z} $

It is noted from these equations that

$ x+y+z=1 $.