Which of the following colors is NOT one of the primary colors in the Opponent-process theory?

The Opponent-process theory of color vision proposes that colors are perceived in terms of opposing pairs: red versus green, and blue versus yellow. This theory suggests that certain colors cannot be perceived simultaneously, as their opposing mechanisms inhibit each other.

In this context, red, green, and yellow are foundational concepts in this theory, as they represent the primary colors in the opponent-process system. Yellow is associated with the combination of red and green perception in the opponent pairing. Pink, however, is not represented as a primary color in the Opponent-process theory. Instead, pink is a tint of red created by adding white, and it does not have a direct role in the foundational pairings of the theory.

Understanding the key colors linked in the opponent-process theory helps clarify why pink, being a derivative color rather than a primary color, is the correct answer in this scenario.