Why does a bull chase something red?

Answer 1

The bull chases the movement not the colour. The bullfighters cloak is traditionally red to a) hide the blood and b) part of the whole spectacle. But it is the origin of the phrase, "Red rag to a bull"

Answer 2

Many people think that red angers bulls. The reason for this is because traditionally bull fighters have used red to get bulls to charge. However, it is not the color that angers them, it is the movement of the waving material that causes bulls to attack.