This is a easy tutorial on how to customize a cop vehicle. This dont work for all police vehicles only a hand full of some. A for sure one I know this works for is the Sheriff suv which you can find a tutorial on how to get that by me on youtube or here on ttg. First when you get a cop vehicle which you can customize drive next to a customize shop and save the game as you are in the vehicle. Reload the game to the newest save you just made and hurry and drive in the shop. It will not only let you pick a new color but it will give you a list of 50 rare hidden ones from gold to diamond looking colors very cool.