Any color you choose for your garage door will be finished with a paint that is designed to be painted over with custom color exterior house paint. If a garage door is not white it is more than likely to have been painted over when the exterior of the home was painted. The only thing you have to remember when painting your garage door is cleaning the surface.
You will have to go to a hardware store and ask someone in the paint dept. that you need something to clean oil and dirt off of your garage door before you paint it. And then closely follow the instructions for that product. If you do not take this step the paint will most likely not stick as well and the paint will bubble up or chip off. The easiest and fastest way to paint a garage door is a professional spray rig. Using a roller or a brush takes a little bit more time because the first coat never covers evenly, and always needs 1 or 2 more coats.
I once installed a white garage door for a customer and he told me that he was going to have someone paint the door for him. I didn’t think much of it until I returned a few days later to install his door opener. When I pulled up I saw the painter just finishing up. I introduced myself and I said you must be the painter. He said no I’m an artist, and a good one at that. He painted the door to look exactly like a Redwood grain door and I couldn’t believe how real it looked. I was amazed.