With this mod you can quickly change all 4 car colors. In the original game, only Camper, Cement Truck and Squalo use more than two colors, so this script is more likely to be useful for other models.
Controls: while sitting in the car, press CTRL + SHIFT + P, then use the NumPad numbers to change colors (8 - for the first color, 4 - for the second, 1 - for the third, 7 - for the fourth, see menu).