The color red does not make bulls angry. In fact, bulls are partially color blind compared to healthy humans, so that they cannot see red. According to the book "Improving Animal Welfare" by Temple Grandin, cattle lack the red retina receptor and can only see yellow, green, blue, and violet colors. Color vision in mammals is accomplished by a collection of cone cells on the back of the eye (the retina). There are three kinds of cone cells: one kind that detects predominantly red colors, another kind that detects mainly green, and the last kind that detects mainly blue. Although cone cells respond most strongly to their main color, they can still respond to other close colors. This color overlap of the cones' sensitivity is what allows us to see so many colors. For instance, a pure yellow color stimulates both the red cone and the green cone, and we experience the combination as yellow. If instead of looking at a pure yellow dot of light, you looked instead at a red dot very close to a green dot with the right balance, you would still experience yellow because the red cones and green cones are being stimulated in the same way. This fact makes the manufacture of computer screens very practical. Instead of implanting a million pixels into a computer screen at every point, each with a different color, the manufacturer only has to construct a grid of red, green, and blue pixels. Humans are actually looking at an array of red, green, and blue dots on a screen but perceive millions of colors. Most mammals, including bulls, are dichromats. This means that they only have two different kinds of cones, as opposed to the three in humans. Bulls lack the red cones, but still have the green and blue cones. A bull's vision is very similar to the vision of a human with red-cone color blindness, known as protanopia. To them, a red cape looks yellowish-gray. It is perhaps the threatening, waving motion of the matador's red cape that enrages a bull, and not the color.