The backstory is all in the movie: Volcanion and the Mechanical Marvel. Though I don't remember much of the movie now, I'll tell you what I do know:
Magearna was made by an inventor as a gift to a princess. At that time, centuries ago, Magearna was painted like a pokeball, it's Original Form. Over time, the paint wore off Magearna, revealing the metal underneath.
So, basically, regular Magearna is just Original Color Magearna with no paint.