In canon, they're green humanoid monsters wearing hooded robes,

Um, what? Description from Prisoner of Azkaban, emphasis mine:

"a hand protruding from the cloak and it was glistening, grayish, slimy-looking, and scabbed, like something dead that had decayed in water; the thing beneath the hood, whatever it was, drew a long, slow, rattling breath, as though it were trying to suck something more than air from its surroundings."

Nowhere does it say that they are green. The best description I can think of is "flying corpses that hide their appearances," which works pretty well as a symbol of Death.