What is the difference between the trichromatic theory and opponent-process theory of color vision?

The trichromatic theory, proposed by Young and Helmholtz, states that the retina contains three types of cones sensitive to red, green, and blue light, and all other colors are perceived through combinations of these three. This explains color mixing and certain types of color blindness. The opponent-process theory, proposed by Hering, states that color is processed in opposing pairs: red versus green, blue versus yellow, and black versus white. This explains afterimages and why we never see reddish-green or bluish-yellow.