They are not as sensitive as the rods so they only work in bright light. There are three types of cones, one for each of the three main colors we see, red, green and blue. Some people have a genetic defect that makes one or more of the cones fail. This condition is known as color deficiency. You may have heard it called color blindness.
Color blindness is fairly common, affecting about nine percent of all humans.
It is much more common in men than in women. To test for color blindness a special picture called an Ishihara test is used. If you jump to our color test page you will be able to test yourself and also experience another interesting phenomenon of our color vision. We know this by looking at how many kinds of cone photoreceptors they have. Another good indication of what an animal can see is by looking at their own colors.