Assuming you are talking about a traffic stop in Georgia (because that is where you are), Georgia police, by law, are not permitted to make traffic stops unless with a marked vehicle, with emergency lights, with contrasting lettering signifying police car, and the officer is in uniform. This is not a constitutional requirement so you can't claim a violation of your constitutional rights should the police violate this law. However, the police do attempt traffic stops with unmarked cars occasionally. It would then be up to a skilled lawyer to make an issue of this in court by arguing the evidence obtained after the traffic stop should be excluded from a trial. Also, of course should you disregard an attempt to stop your vehicle by what you perceive may be an unmarked car, you will run the risk of consequences ranging from being charged with fleeing from the police to being pushed off the road by the police, possibly resulting on your death.
An unmarked car cannot stop you for a traffic violation in Georgia. However, if they observed behavior that gave them suspicion to believe that you were committing a crime other than a traffic violation, they can pull you over for questioning. For instance, if they observed you drive up to a known drug dealer and make what appeared to be a hand-to-hand drug transaction, they could stop you to determine what was going on.