The Short Version
This is a well-known myth. Bulls have dichromatic vision and cannot perceive red the way humans do — it likely appears as a dull brownish or yellowish shade to them. Controlled experiments, including those by MythBusters, show bulls charge moving objects of any color equally and remain calm when objects are stationary. It is the movement of the matador's cape, not its color, that triggers aggression. The red cape is a tradition for human spectators, not a stimulus for the bull.