substances, that is, in undergoing a kind of chemical
disintegration under the action of light-rays, are supposed to
give the sensations white, red, or yellow respectively, and in
renewing themselves again to give the sensations of black,
green, and blue. The dissimilating process seems to bring
about stronger reactions on the physiological side, as if it
were a more exciting process. Thus it is found<1> that as
measured by the increase in strength of the hand grip under
the stimulation of the respective colors, red has particularly
exciting qualities, but the other colors have an analogous
effect, lessening, however, with the descent from red to
violet. The pleasure in bright red, or yellow, for instance,
may thus well be the feeling-tone arising in the purely
physiological effect of the color. If red works like a trumpet
call, while blue calms and cools, and if red is preferred to
blue, it is because a sharp stimulation is so felt, and so
preferred.

<1> Ch. Fere, _Sensation et Mouvement_, 1887, p. 80.

The question of the demands of the eye in color combination is
still more complicated. It has been traditional to consider
the complementaries black-white, red-green, blue-yellow, and
the other pairs resulting from the mixtures of these as the
best combinations. The physiological explanation is of course
found in the relief and refreshment to the organs in successive
alternation of the processes of assimilation and dissimilation,
and objectively in the reinforcement, through this stronger
functioning of the retina, of the complementary colors
themselves. This tendency to mutual aid is shown in the