What do you think of this fact?
“Like a red rag to a bull” is what we say when something instantly sets someone off. The image comes from the bullring. Awkwardly, the bull can’t really see the color at all.
Colorblind to red
Cattle have fewer types of color-sensing cells than humans. They can’t tell red apart from other colors. A bright red cape is about as exciting to a bull as a gray one.
What actually sets it off is movement. You can see this in the bullfight itself: in the first stage, the matador swings a large cape, the capote, which is magenta on one side and yellow or blue on the other. The bull charges it just as furiously as it later charges the small red cape, the muleta.
The three-flag test
In 2007, the MythBusters tested this with a real bull. First, they hung three flags in the arena: one red, one blue and one white. The bull attacked all of them, whatever the color.
Next, they set up three dummies dressed in red, blue and white. Again, every one got knocked over, and the red one was actually hit last. In the third test, a person in red stood completely still in the arena while cowboys in other colors moved around. The bull ignored the man in red and chased the ones who were moving.
Bonus fact: So why is the muleta red? It’s used in the final stage of the fight, and red helps hide the blood.
The color is there for the audience, not for the bull. Standing in front of a bull in red doesn’t put you in any more danger than standing there in blue. What matters is what you do. Frantic waving works, standing still mostly doesn’t.
The saying sticks around anyway. It just sounds better than “like a wildly waved rag of any color”.