One of these threads about the paintshop. sigh, there is no paint shop in the game and the only time where you can choose the colour of your car is when you are purchasing it.
You've got to do the La Sarthe II 24 hour race in an unmodified midget II. You'll be last but, if you complete the race in 100% Aspec, you'll get access to the paintshop from the home page.