Matador's capes may be any color--it is the movement that enrages.
Bullfighters traditionally use red capes. This is probably the origin of the supposition that red enrages bulls. While waving red capes give human audiences a spectacular show, it is the motion that attracts the bull's attention.
Myth
The Discovery Channel's "Mythbusters" television show addressed this question using matador dummies with different color flags. Held still, all colors received equal, minimal reaction from the bulls. Waving elicited charging. Waving white and blue flags aroused the bulls as much as red. Red clothing elicited no more interest than other colors--bulls charged whichever person moved. Mythbusters concluded that the motion of the matador's flag, not the color drives bulls to attack.
Science
Cattle's eyes have 2 types of color receptor cells where humans have three. Bulls have limited color vision, poor ability to differentiate colors and are red-green colorblind. Their color sense is insufficient to react angrily to any particular color.
Fun Fact
The first recorded usage of "like a red rag to a bull" was in 1873. For a hundred years prior, the phrase was "red rag to a pheasant," or to a turkey; a viper--or even a Frenchman. It seems the significant aspect was not the color of the rag, but the excitability of the creature facing it.
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