9. The vehicle has unapproved lens or plastic covers, any other materials that are not original equipment or any colored material placed on or in front of rear lamps, license plate lamps and rear lamp combinations.
that is what is wrong, it is up to each person to determine if it is legal, and after reading that I would say anything that changes the look of your stock light is illegal. The probably either got in trouble, or you got a guy who goes by the law as written.