Opponent process
The color opponent process is a color theory that states that the human visual system interprets information about color by processing signals from cones and rods in an antagonistic manner.  The three types of cones have some overlap in the wavelengths of light to which they respond, so it is more efficient for the visual system to record differences between the responses of cones, rather than each type of cone's individual response. The opponent color theory suggests that there are three opponent channels: red versus green, blue versus yellow, and black versus white (the latter type is achromatic and detects light-dark variation, or luminance). Responses to one color of an opponent channel are antagonistic to those to the other color.
While the trichromatic theory defines the way the retina of the eye allows the visual system to detect color with three types of cones, the opponent process theory accounts for mechanisms that receive and process information from cones.  Though the trichromatic and opponent processes theories were initially thought to be at odds, it later came to be understood that the mechanisms responsible for the opponent process receive signals from the three types of cones and process them at a more complex level.
The three types of cones, S, M, and L, respond best to short-, medium- and long-wavelength light, respectively.  Information from the cones is passed to bipolar cells in the retina, which may be the cells in the opponent process that transform the information from cones.  The information is then passed to ganglion cells, of which there are two major classes: magnocellular, or large-cell layers, and parvocellular, or small-cell layers.  Parvocellular cells, or P cells, handle the majority of information about color, and fall into two groups: one that processes information about differences between firing of L and M cones, and one that processes differences between S cones and a combined signal from both L and M cones.  The first subtype of cells are responsible for processing red-green differences,and the second process blue-yellow differences. P cells also transmit information about intensity of light (how much of it there is) due to their receptive fields.

# History
Johann Wolfgang von Goethe first studied the physiological effect of opposed colors in his Theory of Colours in 1810. Goethe arranged his color wheel symmetrically, "for the colours diametrically opposed to each other in this diagram are those which reciprocally evoke each other in the eye. Thus, yellow demands violet; orange, blue; red, green; and vice versa: thus... all intermediate gradations reciprocally evoke each other."
Ewald Hering proposed opponent color theory in 1872. He thought that the colors red, yellow, green, and blue are special in that any other color can be described as a mix of them, and that they exist in opposite pairs.  That is, either red or green is perceived and never greenish-red; although yellow is a mixture of red and green in the RGB color theory, the eye does not perceive it as such.
In 1957, Hurvich and Jameson provided quantitative data for Hering's color opponency theory.
Griggs expanded the concept to reflect a wide range of opponent processes for biological systems in this book Biological Relativity (c) 1967.
In 1970, Solomon expanded Hurvich's general neurological opponent process model to explain emotion, drug addiction, and work motivation.
The opponent color theory can be applied to computer vision and implemented as the "Gaussian color model."

# Reddish green and yellowish blue
Under normal circumstances, there is no hue one could describe as a mixture of opponent hues; that is, as a hue looking "redgreen" or "yellowblue". However, in 1983 Crane and Piantanida carried out an experiment proving that, under special viewing conditions involving the use of an eye tracker, it is apparently possible to override the opponency mechanisms and, for a moment, get some people to perceive novel colors:

# Other uses
Opponent processes have been used to explain color vision, pain, touch, emotions, smell, hearing, taste, and balance.  It is basically an idea that for every stimulus there is an opposite neurological organization or structure to neutralize the response generated by the stimulus.