The 5 colors of chromatic dragons are the primary colors of light (red, green, blue), plus black and white for intensity. For metallic dragons, there was initially just gold, and the others were added later to parallel the chromatic dragons. Gold, silver, and bronze are commonly used together as a three-step quality grading - see the first, second, and third place Olympic medals, or the common trend of referring to eras of development for a particular craft or industry. Copper may have been chosen for the weakest variety of metallic dragon because it’s the lowest denomination of currency in D&D. Brass? You’ve got me there, not sure why that one was chosen.
Personally, I prefer my metallic dragons to use the seven alchemical metals: gold, mercury (or “quicksilver”), silver, copper, iron, tin, and lead. For the chromatic dragons, I use albino, ebony, citrine (a sort of yellow-green), and ruby, associated with the four physical elements, the four humors, and the four stages of the Great Work (technically black should be derived from the Latin root, niger, but… yeah for reasons I should think are fairly obvious I’d prefer not to). There’s also a fifth category that’s blue in color and associated with the element of aether, a fifth humor which might best be described as ephemera or ectoplasm, and the metaphysical/spiritual component of the Great Work. But they’re almost unheard of in the material plane, being native instead to the aethereal plane.
