The variables of perceived color are reconsidered critically. Two ways of viewing color are distinguished as modes of perception: (1) perceiving the effect of light entering the eye, as such, and (2) perceiving the effect produced by an object on the perceived color of light. For the light mode of perception, there are at least four and possibly six or more independent perceptual variables for the general case. For the object mode this number is reduced by at least one. Both modes can be reduced to three perceptual variables by simple restrictions but, in general, the three to which they reduce are not the same.
Four perceptual variables are necessary (and perhaps sufficient) to describe the color perceptions produced by light sources, illumination, and both reflecting and transmitting objects. They each produce part or all of the perceptions of the four-dimensional color-perception continuum produced by a single homogeneous aperture color with a homogeneous surround.
RALPH M. EVANS, "Variables of Perceived Color," J. Opt. Soc. Am. 54, 1467-1469 (1964)