Each car has a total of 3 alternate paint jobs. Go into time trial mode and finish within the appropriate time limit to unlock a new paint job. Finishing 3rd will unlock one, 2nd will unlock two, and 1st will unlock all 3. Below is the required track for each car.
Lightning McQueen - Figure 8 Track
Mater - Desert Canyon Dash Track
Sally Carrera - Radiator Springs Track
Wingo - Interstate Track
Unlock Boss Cars
Simply beat each car in story mode (DJ, Snot Rod, Wingo, Boost and Chick ) to unlock them.
Other Unlockable Cars
Doc Hudson - Finish the first Gran Prix Cup
Sheriff - Finish the second Gran Prix Cup