Why do bulls become agitated when they see something red? - Animal Q and A

Contrary to popular belief, bulls are not actually enraged by the color red, but are provoked by the movement of the matador's cape, regardless of its color. Bulls are color-blind to red and instead perceive the movement of the cape as a threat or a challenge. The color of the cape is not what causes the bull to charge, but rather the actions of the matador.

Their aggression is provoked by the movement of the cloth, known as a cape or muleta, used by the matador during a bullfight. The waving motion of the cape irritates and confuses the bull, which causes it to charge at the cape in an attempt to defend itself.

* For more understanding about the question, consult a professional bull handler or an animal behaviorist.