Neil, when I answered you original post, I truly wasn't trying to be a smart-alex. I am aware that most colors are imparted onto aluminum during the anodizing process. However, there are other ways to color aluminum—with ink.
I have a friend who used to be a sales rep for a company specializing in ink especially formulated for aluminum. I ask him how it was applied to objects which are odd shaped—not flat like paper. I'm told the process is similar to powder coating, but a bit more precise because the ink isn't self leveling like powder coating. I'm not in the business, and I don't know the exact process used. Nor do I know the costs involved, or the durability. I fostered it only as a suggestion.
I apologize if I came off otherwise.