Really easy, can be done after acquiring your first crib.
Just go on the street, steal a car, and drive it into the garage at the crib. You should be prompted to press Y to add car to garage. After storing the car, use your garage, select the car you just hijacked, and there should be an option to modify the car. The easiest way is to just change the color, as it is very cheap.
Repeat 9 more times.