The trichromatic nature of color vision will enable almost any color to be matched by a mixture of three colors. This trichromacy of vision is also linear. This means that colorimetric equations has properties of an ordinary equation.
The colour matches of quantity Cs of a stimulus S can be expressed as:
Cs(S) = C1(l1) + C2(l2) + C3(l3)
where li are the three primary colors (not necessarily monochromatic spectral colors) and Ci are tristimulus values.
Tristimulus values represent the quantities of each of the three primaries necessary to achieve a match for color and luminosity (figure 1). They can be expressed in units of luminous flux or radiant flux, or even an arbitrary scale provided that the arbitrary scale conveys the relative proportions of the three quantities.
Figure 1. Subjects are asked to adjust three colors on the left hand side of the bipartite field, to match a standard color presented on the right hand side of the bipartite field