There is all sorts of colored carpet out there for cars. The best thing to do would be to find a good customize shop in your area as they will have a bunch of samples for you to look at. A lot of custom interior shops also do custom sound, as they fab their own boxes. So you can check out a couple of them also.
Were you thinking of going with something to match the color of your car? It would look cool to do a mix of paint color match with another color, maybe black. Or a lot of shops can also do custom floor mats which may be a way to bring color into the car.