At the 'Rewards' menu select the 'Enter Code' option and input one of the following case sensitive codes (without the quotes) to unlock the corresponding paint scheme.

Unlock FOX Sports Paint Schemes:
Enter 'foxsports'

Unlock SPEED Channel Paint Schemes:
Enter 'speedtv'

Unlock 3 Custom 'Create-A-Car' Paint Schemes:
Enter 'GoDaddy'