Yes. You can paint a garage door depending on the material. It is not recommended that you paint vinyl material. If you have wood, steel, or a type of metal, you can paint it with a weather resistant paint.
Before painting your door, you need to clean it with a mild soap. Tape off the doorway to keep overspray from getting on the wood. Prime it with a spray paint primer or use a roller. Allow the door to dry for 24 hours before you put the color on it.