What would a dragon's humanoid form look like? I imagine it would be similar in some way to their true form such as a gold dragon having blond hair.
Quite likely, though it could be deceptive. Dragonlance is a good start here; Silvara the wild elf with her silver hair was a silver dragon, but Fizban the crazy old wizard with his mostly-white hair was the one and only platinum dragon.
By RAW? Their appearance is always that of a generic member of the type assumed, per the Polymorph rules. However, that's rather un-fun, so I usually add a bit of flair as a GM.
In the past, I've described them as being an impossibly perfect human man/woman with eyes/hair the color of burnished bronze/glowing copper/glistening gold/pure silver etc. If a character succeeded their perception check to see through the dragon's Disguise DC, I told them that the creature's shadow looms and flickers for a moment, revealing the outline of a serpentine creature with massive wings.
Frankly, I'd imagine even a disguised dragon has the ego of a nigh-immortal apex predator. They'd never stoop to being common looking because they're a f--king dragon and you cross them at your peril.
I like the idea of each one having a "signature" form. but of course they could pick a new one if need be. Also why don't all dragons have a humanoid form? I had the idea of having a copper dragon who's humanoid form would be a beautiful woman with red hair and green eyes.
