There are several quality levels and coating grades of metal roofing. For many owners of warrantied metal roofs installed in recent years, re-painting is not a decision they'll need to make in their lifetime. But take a drive through the country and chances are you'll see agricultural buildings in need of a freshly painted roof. These roofs are older and pre-date the technology incorporated into modern metal building materials.
Yes, you can re-paint a metal roof. If you are doing it yourself, follow the preparation directions given by the paint manufacturer. When you're done, it'll look fresh and new... and there aren't many roofing materials you can "rejuvenate" like that!