Honestly., I'm as surprised as you are how closely D&D books and materials have adhered to the "only bronze, silver, and gold dragons can naturally polymorph" paradigm. It seems a really odd thing to hold as a sacred cow for so long.
Especially since this "rule" doesn't seem to be advertised anywhere. It would be one thing if "everybody knew this" or if it was specifically called out in the Monster Manual ("of course, the gods took the ability to shapeshift away from the chromatics in the year -1234 when Tiamat abused the ability to steal candy disguised as a small child")
Most "rules" of D&D are rather prominently displayed, and then broken anyway. What's so sacred about this one that the unwritten compact to never violate it apparently remains unbroken after three decades of splatbooks?
Okay, here's a canonical Realmslore example: Anwir Duprestiskava, longtime Supreme Potentate of Var the Golden, is an ancient shapeshifted blue.
(Okay, the text doesn't say "shapeshifted," but it does say "playing at being human," and that almost no one realizes what he really is, even though he's been on the throne for over two centuries, because I guess they take a lot of weirdness for granted in the Shining Lands.)
My source is the 3e The Shining South sourcebook, but I think there might be an offhand mention in the 3e FRCS as well.
It is. I really appreciate the effort, and want to thank you very much for going the extra mile.
Sadly, there is no reason Anwir needs to actually appear as a human, since he's described as a very reclusive and very powerful ruler. Exactly the kind of monarch that doesn't do public appearances and isn't questioned for it.
Besides, the text specifically says his closest advisors know the truth. Sure he could have told them, but it's equally likely he had to let them know.
None of this is meant to diminish your research. It just means we have yet another example that appears promising but when you look closer you realize it has been carefully worded to not contravene an unwritten assumption "no evil free shapeshifting"...
I could be wrong, but I thought shapechanging chromatics were fairly common in 3e-3.5e (didn't play that addition so I'm not 100% sure). I know I read some of the dragon based novels during that time that had shapechanging chromatics ( a black in particular).
You would think so, but this thread suggests otherwise.
I would love if you could be more specific on those novels, though.
In 3e it would be easy to pull off simply by making a feat chain (because you'd need a chain of 2 or 3 to be able to purchase that sort of ability in 3e) for draconic shapechanging. Use the 3e Draconomicon feats as examples of the kinds of stuff dragon-enhancing feats can do (conclusion: a lot), and voila! enough information to feel like you are at least in line with published material, even if expanding it.
5e doesn't give monsters feats, so it's harder for me to feel comfortable doing (as a guy who likes to stay canon as much as possible). I would give a dragon class levels, however. And if you allow what they gain from that to affect their spellcasting from the sidebar (which you might not), if you can ramp their CR up to 27 somehow, they can get 9th-level spells. Of course, you may not be interesting in ramping CR that high.
Anyway, I know that's a little off-topic, since you are looking more for precedent than "how can I pull this off in 5e by the book?" but I thought I'd share some of the things that occurred to me months ago when I was considering this sort of thing.
Thank for this pointer.
Looking through the 3E Draconomicon from 2003, I can't find any such feats. But what a search found was Iyriddelmirev, a black dragon described as such: "she is found in the city above, polymorphed into the form of a black-skinned human woman."
She gets this ability from being a 9th-level sorcerer, so it's a spell, not an innate ability. (Of course, we have already concluded the level 4 spell Polymorph doesn't cut it in 5th edition and not only because it's restricted in creature types)
In this book, the ability I would like to see the most (on a chromatic) is "Alternate Form". This is basically the Polymorph spell (which in this edition is enough for humanoid shapes).