If the very idea of bullfights makes you see red, you’re not alone — even though bulls themselves can’t actually see the color. As is the case with other cattle and grazing animals such as sheep and horses, bulls' eyes have two types of color receptor cells (as opposed to the three types that humans have) and are most attuned to yellows, greens, blues, and purples. This condition, a kind of colorblindness known as dichromatism, makes a bullfighter’s muleta (red cape) look yellowish-gray to the animals.
So why are bulls enraged by the sight of matadors waving their muletas? The answer is simple: motion. The muleta isn’t even brought out until the third and final stage of a bullfight. The reason it’s red is a little unsavory — it’s actually because the color masks bloodstains. In 2007, the TV show MythBusters even devoted a segment to the idea that bulls are angered by the color red, finding zero evidence that the charging animals care what color is being waved at them and ample evidence that sudden movements are what really aggravate the poor creatures.
One in 12 men are colorblind, while only one in 200 women are. That’s due to the fact that the red-green variant of colorblindness (in which people have trouble telling red, green, and sometimes other shades apart) — which is by far the most common type — is usually passed down via genes located on the X chromosome. Men only have one X chromosome and women have two, and in women, both X chromosomes need to have the relevant genetic issues for a woman to be born with red-green colorblindness. Blue-yellow colorblindness (confusing blue with green and yellow with red) and complete colorblindness (the inability to see any colors), meanwhile, are passed down via other chromosomes and affect men and women at roughly the same rate.