The tristrimulus values of spectral colors (i.e. light of single wavelength) with unit intensity are called color matching functions. Thus color matching functions   are tristimulus values wrt 3 given primary colors, of unit intensity monochromatic light of wavelength . Figure (2.9) shows the color matching functions for 2⁰ standard observer using primaries of wavelength 700 (red), 546-1 (green) and 435-8 (blue) with units such that equal quantities of 3 primaries are needed to match the equal energy white E.

 For test color with spectral energy distribution  the tristimulus values can be found using color matching function as:

	(2.1)

This implies that two colors with spectral distributions match if and only if

and

where (R₁, G₁, B₁) & (R₂, G₂, B₂) are tristimulus value  of two distributions S₁() and .

       This phenomenon of trichromatic matching is easily explained in terms of the trichromatic theory of color vision that is if all colors are analysed by the retina and converted into only three different types of responses, the eye will be unable to detect any difference between two stimuli that give the same retinal response, no matter how different they are in spectral composition. One consequence of the trichromacy of color vision is that there are many colors having different spectral trichromatic theory of color vision. That is, if all colors are analyzed by the retina distributions that nevertheless have matching tristimulus values. Such colors are called metamers. This is shown in Figure (2.10).  Colors with identical spectral distributions are called isomers