Go along with your car to the Traffic Police station. They will inspect your car and make an entry into their system. They will call you after one week when they will issue you a paper that would allow you to repaint the car. The charges will be QR 100 to be paid through ATM. For the procedure on the changing the color of the car, you will need to ask them and they will advise you of the requirements. There are many garages in the industrial city that do painting of cars and you will need to do some running around to get the cost.