There are a number of cars in the game that can be modified at mod garages located throughout San Andreas. There are a large number of modifications available for cars that can be modified, including custom paint jobs, wheels, and more. There are three different mod garages in the game; TransFender, Loco Low Co., and Wheel Arch Angels.
TransFender offers modifications for a large number of vehicles, pretty much any 4-wheeled vehicle that isn't a lowrider or street racer. Loco Low Co. only offers modifications for lowriders, while Wheel Arch Angels only offers modifications for street racers.
All vehicles are given an option of a solid color change to the outside body for $150, with some vehicles having the option to paint other parts for an additional $150, such as a racing stripe, interior, etc. Note that using a Pay N' Spray will negate any custom paintjob done to a vehicle (only for lowriders and street racers).