Upon starting a Clear New Game from a clear save after beating the game, go to the Official Shop. Any Shinki you buy from now on will allow you to choose up to three alternate color schemes.
Unlock New Club Valhalla
Finish the game by defeating the F1 Champion and save your clear data, but instead of starting a Clear New Game, choose to continue. You'll receive an e-mail informing you that Club Valhalla has reopened under new management, and earlier rivals from the Game Center can now be found here.