In order to do this, you must be playing on Xbox Live. In the room, Pick any color car you want outside of the class of cars you want. Then you exit out of the game and go to single player and go onto ghost challenge, pick any race and then pick the car you want. Don't select it all the way, highlight it and then when it ask for automatic or manual transmission, back out of there and go back to the room. You will now have the color car you want on your favorite car.