Like a particular color on your wheels? When you are on Xbox Live either host or join a game. Then select the car you want to change the color of (example, Porsche 911 GT1 white).

Then select a car from another class and in the color you want (example, Golf R32 blue). Hit the red button (B) until you get to the screen that says Single Player, go into Kudos Challenge and go to the area where your first car is (the Porsche 911 GT1 white in the example) and hit the green button (A) until you get to the screen with the little picture of the car on the top right of the screen.

Now go back into your created game of join a game on xbox live and your car will the be color you chose. (the example Porsche 911 GT1 white will now be blue).