In scientific tests, cats have responded to the colors in the purple, blue, green and yellow range. Red, orange and brown colors appear to fall outside the cats color range and are most likely seen by them as dark to mid shades of gray. Cats appear to see less saturation in colors than humans, meaning cats do not see colors as intensely or vibrantly. Purple, blue & green appear to be the strongest colors perceived by cats.

there are two types of light receptors in the eye. Gray receptors and color receptors, and Cat's have more grey receptors (they are sensitive to motion, detail and low light). Color receptors need more light to work. If one only had color receptors, you would see the world as a blur of colors, and some people who are born with grey receptors only (a rare form of color blindness, see the world as we see black and white movies and photos. Most color blind people are just missing some color receptors and still see some of the color. Also complete color vision with grey vision requires a lot of brain that only humans have.

As wickedwillie mentioned, studies have shown that there are certains colors that cats react to and others that are outside their spectrum, however cats do react to contrasting colors. For instance, we play with our cat sometimes using a laserpointer that emits a red beam. The color red is a color that most cats to not react to as much but when it is contrast to a lighter color (say a white wall or beige carpet) she will react to it. Its not to say that she can indeed see the red point but instead sees the contrasting of colors. Cats usually depend more on movement than color and this just furthers that point. Also on a side note, dogs can be somewhat similar in this matter and actually can be more complex yet more predictable as well. Most dogs don't see many colors but they will react more to some than others. Sometimes by looking at a dog's eye, you can tell what colors they can react to. The type of eyes I have noticed are either redish in tint or green. These different tints provide different results. If the dog has a red tint to his eyes, then he will not see many colors of that spectrum (for instance the same red laserpointer), however if his eye is of a green tint he will see colors nearer to the red color spectrum, and sometimes will also be able to see more colors emitted from television screens and therefore will react to them. I have no idea if the same goes for cats, but it would be interesting to find out.

Keeping in mind that your cat evolved from hunters, you can understand why his sense of sight is one of his strongest. But cats are generally considered colorblind. The retina – the nerve center at the back of the eye – contains cells called cones (responsible for converting light into color) and rods (responsible for black and white). In cats, rods greatly outnumber cones. Theoretically, color perception is possible since the eye contains some cones, but distinguishing color isn't necessary for cats. Brightness is far more important. He can see in the dimmest of lights; his eyes can open about three times as wide as the human pupil and let in as much light as possible at the normal "hunting" times of dawn and dusk.