The sad truth is that if the same bullfighter had a white cloth or a yellow cloth or a cloth of any color, he would get the same thing with the bull. Because the bulls are color blind.
The thing that charge up the bulls is the movement of the cape, and not the color of it. Anything that would wave in front of a bull would excite him. In fact, since the bull is color blind, if a white cape waved in front of his eyes, it would give better result. Because he can see it better, as because bulls are color blind.