Answer: Green
Step-by-step explanation:
Opponent-process theory is the concept that perception of color is based on working of three opponent system.In this system ,three pairs of color were created that were linked with each other :- blue and yellow, red and green and white and black.Activity in a certain color will generate activity in other member.
According to the question, Walter will see green color as per theory of opponent process .As he has experienced minor explosion in chemistry lab which made him see hint of red color, which is linked with green color in pair.Thus, green color would appear as spot.