A cop can legally pull you over in an unmarked car but usually an unmarked police car will not pull you over unless there is something serious going on.
If an unmarked police car is following you and decides you need to be pulled over then they will usually radio for a marked police car to do the actual traffic stop.
If an unmarked police car is trying to pull you over you don't have to stop right away for the unmarked police car due to safety.
You can call 911 and continue on slowly until you reach a populated place for safety.
The 911 dispatcher can let you know for sure if an unmarked police car is following you and wants you to pull over.
Don't take off fast from the unmarked car but just continue slowly and call 911 just to be safe.