Which colour really makes a bull charge?

The idea that the colour red enrages bulls is a myth. Cattle are dichromats, meaning they lack the receptor needed to perceive red, so a red cape looks dull and muted to them, much as it would to a person with red-green colour blindness. What actually triggers a bull to charge in a bullfight is the movement of the muleta, the cape that the matador swishes and waves. Demonstrations have shown that bulls will charge at a waved flag of any colour, and will ignore a red flag that is held perfectly still. So why is the cape red at all? The reason is a very human one: the red colour helps to mask the bloodstains that appear during the fight.