Eccentrically enough, bulls don’t actually mind whether the flag signal in front of them is red, blue, green, yellow, or pink. To them, single color is the similar as another, because bulls are in fact colorblind. It is not the red color that angers them but the action of cape-waving itself. That flapping cape, which only emerges as a dark-gray object to the animal, is very frustrating. Bulls believe that the cape is mockery them, which is why they accuse at it.
Essentially, a red fabric is used because it draws extra thought and effectively disguises any blood that gets sprinkled on it. The yellow color of the capote is merely attractive. If the matador affects the yellow side at the bull, the animal will accuse it also.
