None of those categories are definitive. The chromatics got the yellow, orange, and purple dragons in
Dragon #65 for AD&D 1E, and then
Dragon #248 updated them for 2E.
The ferrous dragons (chromium, cobalt, iron, nickel, and tungsten) premiered in
Dragon #170 for 2E, and then in
Dragon #256 for 3.5E.
The gem dragons used to have
obsidian dragons among their numbers, before their god
Sardior effectively exiled them all.
And there are plenty more where those come from. Mystara has its own collection of (non-psionic) gem dragons, for instance, in the
Monster Compendium - Mystara Appendix for 2E, and there are brown dragons in
FR10 Old Empires for 2E and in
Monster Compendium: Monsters of Faerûn for 3E.
The whole "three groupings of five" is an oversimplification, probably made up by human sages who don't know any better. Heck, even the stats for Tiamat's avatar in
Powers & Pantheons for 2E notes that her five heads can be any combination of chromatic dragons, and lists how her stats would vary if she had a yellow and/or brown head replacing some of the usual colors.
Please note my use of affiliate links in this post.