Cows are mostly color blind. More specifically, cattle are red-green colorblind, so they are unable to see either of those two colors, and their ability to see other colors is limited. Red or green objects simply register as black or grey to a cow's visual perception.
Since cattle have no more than two color receptors, all they see in terms of color are certain shades of blue and yellow. The proliferation of bullfighting in different parts of the world has led to the widespread impression that cattle in general respond to the color red; however, whether the animal is a bull, a cow, a heifer or a calf, cattle cannot see the color red. In fact, when a bull sees a matador's red cape, all it sees is a shade of grey, and a charging bull is responding more to the specific movements of the matador and cape.