I always thought of Dragons as the "ultimate" mortal creature. The Baalor/Solar of the Prime Material Plane, if you will. Each Plane (Elemental, Prime, Astral) has a range of creatures from the lowly to the great, and Dragons were the Great of the PM. None higher, save possibly the Titans (now recast as Elementals, but I'm talking pre-4E here) as peers.
Anywho, for that reason, it always made sense to me that the Dragons who were "of the Universe" would come in variations that reflected various "elements" of the Prime Material Plane (as opposed to the "Elements", the PM uses the Periodic Table). It also made sense to me that Dragons who collected and consumed treasure would over time (or through preference) come to resemble the treasure they ate (you are what you eat, you know).
None of that was ever canon, of course, but that's what I always thought.
I voted Gold, Silver, Copper, Brass and Iron. Adamantium Dragon is too Marvel.