If we gaze for some time at a green square and then close our eyes, we see, as an afterimage, a red square. If we look at a red square, the afterimage is a green square. This experiments may be repeated with any color and the afterimage always turns out to be of the complementary color. The eye posits the complementary color; it seeks to restore equilibrium of itself. This phenomenon is referred to as successive contrast.
In another experiment we insert a gray square in an area of pure color of the same brilliance. On yellow the gray will look gray-violet; on orange, bluish gray; on red, greenish gray; on green, reddish gray; on blue, orange-gray; and on violet, yellowish gray. Each color causes the gray to seem tinged with its complementary. Pure chromatic colors also have the tendency to shift each other towards their complements. This phenomenon is referred to as simultaneous contrast.
Successive and simultaneous contrast suggest that the human eye is satisfied, or in equilibrium, only when the complemental relation is established... Physical mixture of a color with its complementary color yields the sum total of the colors, or white; pigmentary mixture yields gray-black.
... medium gray generates a state of complete equilibrium in the eye. Hering shows that the eye and brain require medium gray, or become disquieted in its absence.... Harmony in our visual apparatus, then, would signify a psychophysical state of equilibrium in which dissimilation and assimilation of optic substance are equal. Neutral gray produces this state. I can mix such a gray from black and white, or from two complementary colors and white. Or from several colors provided they contain the three primary colors yellow, red, and blue in suitable proportions... Two or more colors are mutually harmonious if their mixture yields a neutral gray.
Goethe writes on the subject of harmony and totality: "When the eye beholds a color, it is at once roused into activity, and its nature is, no less inevitably than unconsciously, to produce another color forthwith, which in conjunction with the given one encompasses the totality of the color circle. A particular color incites the eye, by a specific sensation, to strive for generality. In order, then, to realize this totality, in order to satisfy itself, the eye seeks, beside any color space, a colorless space wherein to produce the missing color. Here we have the fundamental rule of all color harmony."
— Johannes Itten, The Elements of Color (1961)