It's legal here, there's no requirement for an emergency vehicle to be gov't owned. I don't know anyone other than a reserves-only department where anyone has bought a car for their "on duty" ride. Special deputies and reserves often buy a CVPI and dress it up to do construction zone details, etc. Most are just flashing rear reds/yellows, which aren't controlled here. No forward facing flashers other than white, no rear flashers other than red/yellow and there's nothing special required. Forward facing red is reserved for emergency vehicles, blue can also be used by volunteer firefighters in their personal cars, and green for volunteer EMTs in their personal vehicles. Why we need to distinguish between EMTs and FFs to that level, I've no idea.
Funeral escorts used to be able to use blue/red, but that was changed awhile back. I forget what they are limited to now.
Oh, and we have take homes. I can pay a set fee and use the car for off duty if I want. I don't, because I don't work any part time and I don't use my car for personal use off duty. I'd rather buy my gas then deal with non-emergency police stuff off duty or put my family in a marked car.