Dyeing carpets is straight forward, Matt. Dye is easily available on eBay. You will need to remove the carpet from the car, so it's a little time consuming. Hoover the carpet, give it a huge blast with a pressure washer and some detergent and leave it for a while to completely dry. Then you can apply the dye (very messy job!) and may need a couple of coats. Depending on what the carpet is made of will determine how we'll the dye will take. Nylon can be dyed but leaves the surface a little coarse got he touch. If this happens, brush the dyed carpet down when it's dried and then add more coats until you're satisfied.