Tercio de Muerte

In the final stage, the tercio de muerte ("a third of death"), the matador re-enters the ring alone with a smaller red cloth, or muleta, and a sword. It is a common misconception that the color red is supposed to anger the bull; the animals are functionally colorblind in this respect: the bull is incited to charge by the movement of the muleta.[14][15] The muleta is thought to be red to mask the bull's blood, although the color is now a matter of tradition.

https://en.wikipedia.org/wiki/Bullfighting