Final answer:
The Young-Helmholtz theory, or trichromatic theory, states that the retina contains three types of color receptors sensitive to red, blue, and green colors. Combined activity across these cones results in the perception of various hues. This theory works in tandem with the opponent-process theory, which applies after the retinal level in the visual processing pathway.
Step-by-step explanation:
According to the Young-Helmholtz theory, also known as the trichromatic theory of color vision, the retina contains three kinds of color receptors. The trichromatic theory asserts that our perception of color is due to the relative activity of three types of light-detecting cells, or cones, in the retina. These cones are sensitive to different wavelengths of light, corresponding to the colors red, blue, and green. While the theory recognizes that there are thousands of hues distinguishable by humans, it explains that these hues are a result of various combinations of stimulation of the three types of cones.
Complementing this theory is the opponent-process theory of color vision, which suggests that color perception is controlled by the activity of two opposing systems: one that pairs red with green, and another that pairs blue with yellow. While the trichromatic theory applies to the level of the retina, the opponent-process theory is more relevant once the signal moves past the retina towards the brain.