In any system which defines color on the basis of the non-spatial, non-temporal properties of light it is shown that white, gray, and black are not colors. The perception of white is shown to be caused by a physical property of the object not associated with its selectivity. This property is the diffusion of light. White is shown to be one extreme of a series starting at clear and passing through various degrees of pale white to a maximum. Gray is shown to be the perception of the relative visual effectiveness of the light from two areas and cannot be seen unless at least two areas are present. Black is considered the extreme in lack of relative effectiveness.
RALPH M. EVANS, "On Some Aspects of White, Gray, and Black," J. Opt. Soc. Am. 39, 774-774 (1949)