Are your original panel molded in the proper colour, or were they too painted?
If they are molded in colour, then use caustic soda, lye, drain cleaner or oven cleaner. They are all basically the same (caustic) and they will melt the paint without damaging the plastic, even if your panels are made from ABS.
Do not use lacquer thinner, because it will damage the plastic or at least fade it. Try to find out what material the panels are made from by locating a stamp on the inside. 99% of plastic products have this stamp.
If the original panels were painted from the factory, then just respray them with vinyl die which can be purchased in spray bombs in the proper factory colour. I've done this tons of times with excellent results.
Give us an idea of what vehicle you're working on as well as colour code if possible.