The color red isn’t what causes bulls to attack. In fact, bulls don’t seem to have any color preference at all.
Actually, it doesn’t. Bulls, along with all other cattle, are color-blind to red. Thus, the bull is likely irritated not by the muleta’s color, but by the cape’s movement as the matador whips it around. In support of this is the fact that a bull charges the matador’s other cape — the larger capote — with equal fury. Yet this cape is magenta on one side and gold or blue on the other.