To understand Hering's theory, it is important to understand Thomas Young and Hermann von Helmholtz's theory, also known as the trichromatic theory. They proposed that the human eye perceives colors based on three primary colors, these colors being red, blue and green.
Hering disagreed with the Young-Helmholtz theory. Instead of color being comprised of three basic colors, he suggested humans perceive color based on six primary colors. The primary colors represent themselves in pairs, opposing one another. The primary color pairs are red-green, blue-yellow and white-black. Since these colors oppose each other, we do not see greenish reds, blackish whites, or bluish yellows.
Afterimages, also known as ghost images, are optical illusions. If a person looks at something then looks away, he sees an afterimage. A common demonstration of afterimages is to stare at a picture of the American flag for several seconds. When you look away from the image, you see a version of the flag in which the white stripes and stars appear black, the red stripes appear green and the blue appears yellow. According to Hering's theory, the colors of the afterimage are indeed the colors he proposed to be opposites.
The theory of human color perception proposes that signals are passed from rods and cones to the visual system in the brain. Two types of cells are present in this process: bipolar and ganglion cells. Visual information collected by the cones is passed to the ganglion cells. Parvocellular ganglion cells (P cells) process the information about color.
Hering's color theory has inspired other theories. Edwin Land's retinex theory (1970) is based around Hering's suggestion that there are six primary colors. This theory proposes that in the brain, three colors are translated to six. Leo Hurvich and Dorothea Jameson's hue cancellation theory is also based on Hering's theory. They suggest that mixing opposing colors would make another color. For example mixing red and green would make yellow, not reddish green.