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.
The Discovery Channel's "Mythbusters" television show addressed this question using matador dummies with different colour flags. Held still, all colours 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 colours--bulls charged whichever person moved. Mythbusters concluded that the motion of the matador's flag, not the colour drives bulls to attack.
Cattle's eyes have 2 types of colour receptor cells where humans have three. Bulls have limited colour vision, poor ability to differentiate colours and are red-green colorblind. Their colour sense is insufficient to react angrily to any particular colour.
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 colour of the rag, but the excitability of the creature facing it.
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- Discovery Channel Myth Busters: Does the Color Red Really Make Bulls Angry?
- Mythbusters Results: Episode 85 Red Rag to a Bull
- Colour Blindness: Are Cats Dogs and Bulls Color Blind and Other Color Blindness Myths
- The Naked Scientists: Science Questions: Can Bulls See Red?
- Phrases, sayings, idioms and Expressions: Phrase Finder: A Red Rag to a Bull