It is not as simple as seeing whether a bull charges a red cape, scientists say, and some of the evidence is indirect.
Indirect research methods include examining the behavior of the animal in its natural world.
One way is simply to look at whether the animal itself has colors.
You can assume that if the animal has color that is meaningful to it in some way, it probably has color vision.
For example, brightly colored bird species may have color differences between sexes, and some primates can tell when the female is receptive to mating by color changes.
Or you can see if the animal uses color choice for food, telling the difference between ripe and unripe food, for example.
These pieces of evidence do not by themselves prove that an animal sees colors.
Another step is to do anatomical work on the animal’s eyes, looking to see if they have cones, the color receptors in the retina that strongly suggest color vision; but this kind of research involves killing an animal.
There is also electrophysiological research, in which anesthetized animals are presented with a color stimulus of a specific wavelength to see if there is an electrical response in the brain.
Then there is laboratory research with unanesthetized animals, in which an animal might be trained to expect a reward from pressing a food bar if it chooses the right color. Such experiments have to be designed so the response is to color, not brightness, saturation, texture or some other visual variable.
It is also possible for an animal to see color but not to attend to it.
However, if you motivate an animal whose brain is wired to be able to make sense of color information, it will react to color. Domestic cats don’t attend to it much, and given their lifestyle, this is not that surprising. They are nocturnal, and go after gray and agouti-colored mice.
Agouti refers to animals that have bands of dull colors on each hair.
Studying feline color vision usually involves putting a cat on a stand about four feet high with a choice of colored squares to and on.
If the cat is rewarded when it jumps onto a red square, for example, it can learn to distinguish it from a blue square. An animal like a monkey might learn to do this in only ten or twenty trials, but cats take a much longer time.
On the other hand, a cat learns quite quickly to distinguish between things like straight and wavy stripes.