Althouhg Lighting Challenges and other stuff unlock some plates,to unlock the paint schemes and the other tracks(a day race at Richmond or night race at Atlanta, etc.) you have to unlock them. In order to do this follow these steps:

1.When in Race Now select medium or higher,custom make work if you set the A.I. to Veteran
2.Select the driver any current paint scheme open already will do.
3.Pick the track(if you want to unlock the other race at the track or both car and track)
4.Win the race to earn car/track/both