Final answer:
The green, yellow, and black flag is related to the opponent-process theory of color vision, which is based on how colors are coded in opponent pairs in our visual system, leading to phenomena like negative afterimages.
Step-by-step explanation:
The green, yellow, and black American flag that Dr. Turner showed in class provides evidence to support the opponent-process theory of color vision. This theory posits that color is coded in opponent pairs: black-white, yellow-blue, and green-red. Essentially, while one color stimulates, the other inhibits. As a result, we do not perceive colors like greenish-reds or yellowish-blues. When staring at the flag with these opponent colors and then looking away at a white surface, the viewer experiences a negative afterimage with the complementary colors due to the activation and subsequent inhibition of these opponent pairs in the visual system.