I just thought i'd let you guys know of something I figured out how to do. When you are not looking at a certain part of the car it appears a solid colour but looking at it you will see a pattern underneath.
The view not looking at it;
When looking at the correct area;
All you need to do is paint one of your cars a "special colour" then create a pattern as a layer group and place it where you want the effect. You then create a bright colour border that covers the outline of your pattern then voila.