Bullfights can be quite bloody affairs, and matadors use red so that the blood of the bull is less visible on the cape. Contrary to popular belief, bulls are not angered by the colour red.
You’ve almost certainly heard the myth that bulls will go crazy at the sight of anything red. For quite a while, popular culture has included stories of unfortunate characters, unlucky enough to be wearing red clothes or carrying a red balloon, suddenly finding themselves the sole target of a bull’s aggression. But not only is this trope overdone, it’s completely incorrect.
These clichés stem from matadors’ use of muletas (red capes) to taunt bulls into an ill-advised charge. However, the cape’s colour is not for the bull, it’s for the spectators. A red cape masks the blood of the bull during the finale of a bullfight when the matador finally puts down the bull and ends the fight. With a red cape, the spectacle looks a lot less violent and bloody than it really is, in the hopes that it will be more enjoyable for those watching.
The cause of the bull’s anger is not the redness of the cape. That would actually be the cape’s motion. A cape of any colour could cause the bull to charge as long as the bullfighter waves it the same way. In fact, bulls are colour-blind. They only have 2 cones (colour receptors) in their eyes, whereas humans have 3.
Bulls, like other ungulates (animals with hooves), have dichromatic vision. They have two different cone cells, those cells in the retina that detect color. One type of cone cell detect short wavelengths (S-cone) that is most sensitive at 444 to 445 nm (blue/violet region). The other type of cone cells perceive medium to long wavelengths at around 552-555 nm (yellowish-green).
Since red light has a wavelength of around 650 nm, the bulls would only be able to see the capes in shades of grey.