Is this a storm door or an entry-type? Have to be careful when choosing a color- if it gets direct sun a dark color can cause it to warp and void any warranty. But no reason you can't do everything yourself. Remove it if possible, sand it, prime any spots that are bare metal and put at least a couple of coats on. Voila, new door.
Is there any rust or is the paint starting to chip that you're painting over?