Most all states have laws concerning colored lights that face forward. Chances are that blue and/or red lights will require the vehicle owner to be a LEO or some sort of emergency responder. Some states even prohibit green lights unless they are used by volunteer firefighters or medic volunteers.
Ex-cop cars are typically sold at auctions and some still have spotlights and push bars left on them. Black-and-whites are sold without repainting; the decals are removed.
It's against the law everywhere to impersonate a police officer and I would bet that even driving an ex-cop car in a manner that would imply you are an LEO would get you a quick visit from Officer Friendly. And he would most likely not be friendly.
At least in CA the lightbar would have to be removed, the doors painted over and the word police removed. The Geek Squad cars used to have white doors but those were painted black with only the roof left white.
I've seen a few x CHP cars at shows. I don't think they were driven on the streets. One was a 1960 Dodge Polara. Original down to the Motorola radio and whip antenna on the rear. Was way cool!! He said the CHP loved him to display it in shows and parades.
Police her use cars like VW Passats... anyway I saw a guy (I think it was a promotional car for something) that had a white Passat with a blue stripe running the side of it (non reflective of course) a roof rack that looked at night like lights.
But the kicker was... police have on the side of the car saying "policija" meaning police but he put on "pozicija" meaning position. At night from a distance it looked convincing....
I laughed so hard I cried...
He wasn't breaking any laws because the blue was not reflective and the sign didn't say police. The yellow light's are ok anyway. But that's here.....I don't know how things are in other places.... the Italians have police cars that look like ice cream trucks....
I saw on a show where a guy recreated the car from transformers. (Maybe that one) and claimed that he's only ever been pulled over because cops were curious about the car but never gave him any trouble.