Lots of unmarked units running around, but they aren't giving out tickets, generally, in California. They are usually hunting felons.
CHP runs low visibility vehicles which are all white and have no light bar. It wouldn't look like a cop car in the rear view mirror.
For this reason, NOT stopping for an unmarked or low vis vehicle could result in way more excitement than most people can handle. Your car may match one that just left a takeover bank robbery and the FBI may lock on to you by mistake. They won't take "No" for an answer. Long before you see any obvious cop cars, your car tires may be spiked or your car rammed off the road. It isn't unusual for LE officers to shoot at (and kill) fleeing felons if the circumstances justify it under Graham versus Conner. Because you ran, they will believe you are guilty of something and you're going to be arrested regardless for failing to stop.
Getting bail may be difficult since you've demonstrated a propensity to flee. So, you get a lawyer and wait in jail for your trial to come up so you can tell a judge that you only stop for marked police cars, on principle.
Judges rule on law, which has supported the authority to make stops without any identifying markings. In California, a forward facing steady red is enough. I'm pretty sure that is in the California Vehicle Code. The feds typically only have that, and their authority trumps state law.