Do you want to know how it works, or how to do it
Basically it works because you first select the color you want, and then the car you want as a separate process
For example, say you want the Enzo Fararri in the same blue as the Lotus Esprit. When you are in an online game, make a note of one of the players or which friends are in it, and then select the Lotus and the color you want
Then back out of the game, back out to the opening PGR2 menu and select the Time Attack solo games. Choose the Car option after that, and change your car to the Enzo Farrari and when it asks you to choose between Automatic or Manual transmission, DO NOT CHOOSE ONE. Instead back out from there, and back again until you see Online Options available with the Yellow button. Press that, and go back to the game you were in by choosing to join one of the players or friends in that game
Voila! You should see your Enzo selected, in the Blue you previously selected with the Lotus. It stays because you never got to the screen where you select the color of the Enzo. So it remembers the previously selected color and the newly selected car
Once you've done this, the car and color will stay until you choose a different car or join a game with a restricted car class.