Final answer:
The opponent-process theory explains why people who are red and green colorblind can still see yellow, as yellow is part of a separate opponent pair, unrelated to the disrupted green-red processing. This theory works in conjunction with the trichromatic theory at different levels of the visual system.
Step-by-step explanation:
The ability for people who are red and green colorblind to still see yellow can be best explained by the opponent-process theory of color vision. According to this theory, color is coded in opponent pairs: black-white, yellow-blue, and green-red. Red-green colorblind individuals have a disruption in the green-red pair processing, yet their ability to perceive yellow is unaffected as it is part of a separate opponent pair, allowing yellow to be seen distinctly. The opponent-process theory complements the trichromatic theory, which applies at the level of the retina where cones are responsive to red, blue, and green wavelengths. Hence, bright yellow can be seen due to the activity of cones and the further processing of color that does not rely on the red-green opponent pair.