While I certainly can't say for sure, I would be very surprised to hear that any state has it in law that police vehicles must be marked. Some agencies may have policies against unmarked vehicles making traffic stops while others may embrace it depending on their approach to enforcement. Agencies that are more focused on prevention through visibility will only do stops with marked vehicles and have fewer unmarked. Agencies with more focus on prevention through the fear of enforcement may used unmarked or subdued marked vehicles for traffic enforcement. Where we are, I see a mixture of both. Local PD has several unmarked vehicles they use for traffic including a Toyota Camry, Toyota Tacoma PU, Ford Escape SUV, etc. All colors and in no way looking like a police car. However, the officers in them are fully uniformed. The agency does (or did) have a policy that plain clothes officers could not make routine traffic stops. Highway patrol uses mainly marked, but they have some unmarked as well as subdued cars.
The use of unmarked vehicles is not entrapment. The lack of official markings does not force a person to speed or break other traffic regulations that they would have otherwise not broken.
Besides, it really doesn't matter if the vehicles are marked, most people are oblivious when they drive anyway. Stopped a car one day who blew by me. I was in the outside lane and he passed in the inside lane on a 4 lane road. Paced him at 30 MPH over the limit. Fully marked Crown Vic with large light bar on top (pre LED bar days). When he stopped, he asked me where I was sitting and how I clocked him since his radar detector never went off...He never saw me when he passed me. A few months later, the traffic judge schooled him on certified speedometers and pacing.