Bulls Are Enraged by the Colour Red
"The belief that bulls become aggressive specifically because they see the colour red, as in a bullfighter's cape."
Cattle are red-green colourblind, so a bull can't physically single out red the way the myth assumes. What actually provokes them is movement. In controlled tests, a matador's traditional muleta was replaced with capes of different colours, and bulls charged whichever cape was being waved, ignoring stationary red cloth entirely. Red was chosen for the muleta for a much more practical reason: it disguises the bull's blood during the fight, keeping the spectacle visually palatable for the crowd, nothing to do with provoking the animal.
Myth vs. FactThe bull is reacting to the cape's movement, not its colour, which it most likely can't even perceive the way people assume.
Sources: Temple Grandin, Improving Animal Welfare; controlled bullfighting-cape tests documented via Snopes and academic sources.