Not sure if this is considered a spoiler since it's got nothing to do with mission progress.
*NOT about FIB vehicles, police bikes, police vans, or other special units*
This topic covers all 4 currently known colors of the unmarked police cruiser.
All in one location!!!
UPDATE: Page 2 reveals a 5th color (Silver) has been discovered!
They can all be obtained during the side quest involving Barry, when you use Franklin to find a weed stash. Once in the area, you'll have just a few minutes before the timer stops & the mission fails. That's the only issue because the unmarked cruisers are only present there during the mission. The police vehicle in the back actually doesn't even appear until the automated phone call is made.
Near the truck (with the stash) are the two unmarked cruisers, DO NOT GET IN THE TRUCK!
Both police vehciles have 2 occupants, a driver & front passenger. The cars are parked.
Normally a gray unmarked cruiser will be sitting out front.
Normally a blue unmarked cruiser will be sitting behind it in the next alleyway.
Throwing the police out of the car is recommended over shooting them.
Shooting them gives you 3 stars, which will call for a helicopter......a pain to lose.
Once you've gotten whichever vehicle you desire, lose the police. There's a big path, shallow water nearby & under the highway, a good spot. Once your wanted level is clear, let the remaining mission time expire, DO NOT RETRY MISSION, instead quit the attempt & continue free roam. You'll remain in the car. Recommend that you save the car in a garage, sometimes saving & loading while in the car itself will cause it to vanish.
For the red & black versions though, it's a bit time consuming.
You'll need to steal the blue version first, proceeding through the steps presented above. DO NOT SAVE THE CAR. Travel just far enough that the mission search area lights up again (green zone). The police cars will not appear if the mission doesn't. Arriving in the blue unmarked police car will cause a red version to spawn where the gray usually appears, & a black version to spawn where the blue usually appears.
In the end, if you want all 4, you'll have to steal the blue version several times. Also you can technically do this however many times that you want, aquiring countless unmarked police cruisers, as long as you don't complete the actual side quest mission that is.