They aren't. Bulls can't actually see the color red. They have a form of color blindness that doesn't let them see that color. They get aggressive toward the waving and flapping motions of the cape and yelling. But the cape could be any color. Red is chosen because you don't see the blood of the bull. They get angrier and angrier in bull fights because the matador is actually stabbing them with a sword when the bull runs by them.