Aluminum isn't hard to paint but you have to be very particular about the process. If you are going to paint aluminum it must be cleaned and then prepped with a chemical conversion coating, often called Iridite or Alodine (trade name). The spec you can call out for the finish is MIL-DTL-5541 Type II, Class 1A. Type I is ok too but is getting harder to find (it contains hexavalent chromium). The color of the chem film doesn't matter. After that just prime and paint. Without chem film prep the paint will just peel off.