Answer: YELLOW; BLUE
Explanation:
OPPONENT PROCESS THEORY OF COLOR
This theory suggests that we are able to perceive colour through 3 receptor complexes who oppose each other being the red-green complex, the blue-yellow complex, and the black-white complex.
According to this theory, there are cells that can only detect the presence of one color in the complex at a time because the two colors oppose one another. For example, humans cannot see a greenish red because the opponent cells can only detect one at a time.
Interesting theory but let me not digress. The blue-yellow complex oppose each other and so the first option is correct.