Per Josef Albers’ description from the “Interaction of Color ” :
When pigment or paint is mixed on a palette or in a container it is seen by the eye as reflected light. This mixing will never result in white as the sum of colors. On the contrary, the more color that is mixed, the more the mixture approaches a dark grey leaning toward black. This we call subtractive mixture. Also, the psychologist, who mixes colors of reflecting colorants optically on the rotating wheel, is not able to arrive at mixtures lighter than the lighter color parent of the mixture. As optical mixture usually means less loss of light than mechanical mixture, the psychologist’s sum of all his colors normally approaches a middle grey instead of the dark grey of the painter.
The conclusion is : mixtures gain light only in direct color, as in (a) whereas mixtures of reflected colors lose light, as in ( b).