Its just painted aluminum. Paint it like you would paint anything else for outdoor use. Lightly sand them with 400 or 600 grit paper. Clean them with a good surface cleaner. Spray with any good quality paint in a can made for metal and the outdoors. Tape it all off. Prior to spraying, warm up a pan of water, not hot, but very warm. Stick the can it in for a few minutes. This improves flow and gives you a more even spray, spray cans normally are not the best way to paint. Optionally you can follow up with a clear coat sealer.