| | Re: why do bulls charge at the color red?
Bulls are indeed colorblind. To a bull's eye, a red cape is a gray cape. The red cape, which inspired the phrase "seeing red," is purely a theatrical tradition.
However, to a bull bred exclusively for aggressiveness, that flapping cape can prove very irritating. As this post from the Washington University Medical School astutely notes, animals bred for bullfighting are tested around the age of three for aggressiveness before they go into the ring. The orneriest ones make the cut.
It's also worth pointing out that the animal in question has already been stabbed several times at the base of the neck. This happens early in the bullfight, and is performed by a gentleman known as a "picador."