Dragons are angels (or gods) and angels(/gods) are dragons. At the beginning of time, Greatfather Io, Whose-One-Is-All, metaphysically slew himself, fracturing into innumerable pieces, some of them of his design. This both created the physical universe and its first denizens, and possibly permitted the existence of individual souls. Of these, the greatest children were dragons like him, and the greatest of them are the gods. Originally, Skythia, Whose-Guile-Is-Power, was the ruler of this pantheon; for dragons, gender is a choice, so it is equally valid to view Skythia as male or female, but generally Skythia seems to have chosen female more often. Thus she was Daughter/Hunter/Queen (but also Son/Hunter/King); and her intentionally-crafted partner was (and is) Arkhos, Whose-Might-Is-Wisdom, generally choosing male aspect, and thus Brother/Scholar/Priest (but also Sister/Scholar/Priestess). Skythia ruled and Arkhos salved; she struck, he studied; she inherited, he protected.
The two of them produced an egg, and prophecy claimed it would give rise to the true successor to Dead Io, the being who would outlast time itself and go on to engender the world that would come after. But the egg was stolen by wicked things, things which wish to see not merely the end of the cycle, but the ruination of all it creates; an eternal torment of the current world. These demons tricked Skythia into thinking Arkhos had destroyed the egg, and tricked Arkhos into thinking the egg was a thing of evil, on the very day that Skythia and Arkhos were to be bound in matrimony. They fought, but Skythia was wild with rage, while Arkhos held his senses, and drove her back.
She has since sworn vengeance for a wrong that never happened, and thus her own mind wars against itself, shattering her into seven colors and sundering her from noble Skythia into twisted Sutela, and tainting her Prismatic brood into fallen Chromatic dragons. But two colors--Yellow and Purple, once Topaz and Amethyst--remain suppressed, for reasons unknown, and no dragon of any of these types has been seen since the Fall. In truth, both Sutela's Yellow and Purple heads are suppressed, for they contradict what the other heads desire, as they seek forgiveness and rapproachment. Thus the Topaz and Amethyst dragons secretly remain, bound and hidden from all eyes, and even Sutela no longer remembers them.
Now Arkhos rules alone, unfit to the task but unable to relinquish it to anyone else, designed as he was to be advisor, not ruler. And the hole in his heart cannot mend, because he was made for Skythia, and cannot help but see his Beloved in the fallen Sutela, no matter how many millennia may pass. Thus is the world marred nigh even unto its birth, because the one meant to rule it has fallen into darkness and corruption. Perhaps, some day, this wrong may be righted. None, save the wicked ones who caused it, know the truth of the Egg and the dark design that wrought such pain upon the children of Dead Io. But no secret is so safe that it cannot be learned, if its secret-keepers still live to exploit it.
TL;DR: Dragons here are angels, the Metallic brood belongs to Arkhos, the Chromatic (fallen Prismatic) brood belongs to Sutela (formerly Skythia). All Metallic dragons are pure metals: Gold, Silver, Iron, Copper, Mercury, Cobalt, and Wolfram. All Prismatic dragons are crystalline: Topaz, Diamond, Onyx, Ruby, Amethyst, Sapphire, and Emerald, corresponding to Yellow, White, Black, Red, Purple, Blue, and Green. Other dragon-gods generally do not have broods (or, if they do, their brood is simply smaller versions of themselves), and feature non-metallic and (generally) non-crystalline materials, e.g. Opal, Pearl, Coral, Jet, Amber, Ivory, Flint, Hyaline (aka glass), etc.