You can paint it any colour you want. It just has to have a few things, like the official SRC number on the sides and roof and an AM or PRO logo on the back.

Paul has a link to blank skins which are helpful for making your own skin from, but you can also just download a compatible skin online and use that if you prefer.

Chapter 2.1 of the rules has a list of the cars which can be used, but many are taken already.

Official Rules

Here is the drivers list, including numbers.
Driver list & Spotters Guide (S3)

Someone has forgotten to update it though, so some numbers are now taken, but that's most of them.