I really liked the Iron and Adamantium Dragons being core. There were designers notes that they got rid of the Bronze and Brass because they were too close to Copper in looks/Style, and wanted the Metallics to all be fairly distinctive.
Having the other return in the Dracomonicon2 was cool as well though - the more types the better. I also liked the Mithral, Steel, Mercury and Orium dragons.
What is Orium meant to be anyway? I tend to think of it as Uranium, but that would be silly. Not that thats stopped me in the past. There's Pink Dragons in my campaign. They incorporate the worst aspects of both the White and Red Dragons, breath a superheated blast of steam, and live in a constant state of near fury, because they're pink.
I've pretty much made Dragons for every color, and every Metal. The Metallics are divided into Noble and Non-Noble Metals, the Noble metallics tending towards good, the non-nobles Neutral.
THe other thing I really liked about 4E Dragons was that the Metallics weren't inherently good. They could be as selfish and unpleasant as the Chromatics, often slightly worse because they often believed themselves to be righteous. I also took it the other way and made some Chromatics not evil as well.