Hastings

You can only use different textures for police cars models. You have Police (Vapid Cruiser), Police2 (Police Patrol in game), Noose Cruiser, Noose patriot, FBI. All those cars have 4 liveries (used to have, at least, they say you can have more now). using '?do=embed' frameborder='0' data-embedContent>> you can assign a different livery to different city area.
 
Example from my game:
 
Police Cruiser:
Livery 1 LCPD, assigned to Algonquin
Livery 2 Northern Gardens police, assigned to Bohan
Livery 3 LCPD HWY, assigned to highways. 
Livery 4 Alderney State police, assigned to Alderney.
 
Same with other cars.
 
So basically you download a car model you like, when you find textures which suit the model (I got mine from several different car mods), and then apply to each area.