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- Create a folder called "skins" inside the original car folder.
- Put your paintscheme into the skin folder
- Load the game, and on the car selection screen pick that car, click skins tab, pick your skin.
NOTE: As you already know custom skins aren't automatically downloaded on online sessions. So nobody will see your new skin (except you) unless you have shared your skin with them.
Of course this method won't generate any mismatches
(Tip by Hugh Harse)