Bulls are color blind. They cannot see the colors red or green.
Like all cows, bulls have deuteranopia, meaning they cannot discern red and green colors and instead see shades of yellow or blue. When it comes to bullfighting, people are often under the impression that the red color of the matador's cape is what angers the bull in the arena, and, symbolically, red is a color that denotes anger or passion. In reality, the fluttering movement of the cape is what agitates the bull and causes it to charge the matador, not the color. The animals are more incited by white or blue capes, but the red serves a particular purpose in the arena: the color masks any blood that the creature may spray while injured.