Painting your fiberglass door is no different than painting any other door, though the preparation is a bit different if the surface has not been smoothed. The main preparation is sanding and giving it a thorough wash, and you can speed things up by removing any inserts or attachments to the door before beginning. Once the door is prepared, you use the same process of priming and painting that you normally would. Regular house paints are perfectly well suited for fiberglass doors, so you will have an almost limitless variety of colors to choose from.
Prepare the Door
Take off whatever hardware you can to give you better access to the door. You can remove the door from the hinges as well, but sometimes leaving it up is easier than removing it and working on sawhorses, as you can get to both sides at the same time.
If the fiberglass exhibits any texture on the surface and you want to smooth it out, give it a good sanding. Use a sanding block or electric sander for best results. A 100-grit sandpaper is a good place to start; follow that up with 220 grit for a fine surface. Clean the door when done with a mild detergent and water, and be sure it is rinsed well. Avoid solvents such as turpentine that can damage the fiberglass, and allow the door to dry completely before proceeding.
Latex paints work well with fiberglass and are easy to work with. Paint the door with a latex primer first. Fiberglass surfaces can get quite smooth, as you will have realized if you sanded the door, so if you want that super clean finish, you can apply the primer with a spray. Rolling or brush application with care also will yield good results. Allow the primer to dry completely before putting on the top coat.
Apply two coats of paint for total coverage unless you are using a pigment-rich paint such as Aura. Reattach removed hardware and remount once the latex paint has set sufficiently that you won't damage the skin of the paint. Be careful: Paint can be dry to the touch but still wet underneath. Refer to the manufacturer's instructions if you have any questions.