If bulls are colour blind, why use a red cape?

The red cape is more for the people watching then the bull. The bull just reacts to the movement.

This is what I read: “The cape is thought to be red to mask the bull’s blood”
Care of Wikipedia. Not sure if that is the best source but it seems like a good answer. I mean it is nasty what they do to them bulls.

It’s better theatre than a grey cape.

Welcome to Fluther, @TheRealSneakers

I was glad when I found out that there are different kinds of bullfighting, and not all of them utilize capes, and not all of them result in the death of the bull. In some forms they are retired and used as breeders. That seems more humane, but I wish the whole practice would die.

The waving is to attract and irritate the bull, the color is to disquise the blood when the bull notices.