Opponent-process theory is a psychological and neurological model that accounts for a wide range of behaviors, including color vision. This model was first proposed in 1878 by Ewald Hering, a German physiologist, and later expanded by Richard Solomon, a 20th-century psychologist.

The opponent-process theory was first developed by Ewald Hering. He noted that there are color combinations that we never see, such as reddish-green or bluish-yellow. Opponent-process theory suggests that color perception is controlled by the activity of three opponent systems. In the theory, he postulated about three independent receptor types which all have opposing pairs: white and black, blue and yellow, and red and green.

These three pairs produce combinations of colors for us through the opponent process. Furthermore, according to this theory, for each of these three pairs, three types of chemicals in the retina exist, in which two types of chemical reactions can occur. These reactions would yield one member of the pair in their building up phase, or anabolic process, whereas they would yield the other member while in a destructive phase, or a catabolic process.

The colors in each pair oppose each other. Red-green receptors cannot send messages about both colors at the same time. This theory also explains negative afterimages; once a stimulus of a certain color is presented, the opponent color is perceived after the stimulus is removed because the anabolic and catabolic processes are reversed. For example, red creates a positive (or excitatory) response while green creates a negative (or inhibitory) response. These responses are controlled by opponent neurons, which are neurons that have an excitatory response to some wavelengths and an inhibitory response to wavelengths in the opponent part of the spectrum.

According to this theory, color blindness is due to the lack of a particular chemical in the eye. The positive after-image occurs after we stare at a brightly illuminated image on a regularly lighted surface and the image varies with increases and decreases in the light intensity of the background.

The veracity of this theory, however, has recently been challenged. The main evidence for this theory derived from recordings of retinal and thalamic (LGN) cells, which were excited by one color and suppressed by another. Based on these oppositions, the cells were called "Blue-yellow", "Green-red" and "black-white" opponent cells. In a recent review of the literature, Pridmore notes that the definition of the color 'green' has been very subjective and inconsistent and that most recordings of retinal and thalamic (LGN) neurons were of Red-cyan color, and some of Green-magenta color. As these colors are complementary and not opponent, he proposed naming these neurons as complementary cells.

A-process refers to one of the emotional internal processes or responses of the opponent-process theory. The A-process is largely responsible for the initial, usually fast and immediate, emotional reaction to a stimulus. The theory considers it a primary process which may be affectively positive or negative, but never neutral. The theory also proposes that this process automatically causes a B-process, which is subjectively and physiologically opposite in direction to the A-process.

There is a peak response to any emotional stimulus which usually occurs rapidly, usually out of shock, but lasts only as long as the stimulus is present. In a physiological sense, the a-process is where the pupils dilate, the heart rate increases, and the adrenaline rushes.