All the hundreds of fandoms and headcanons aside, “James Potter was black” is still my favourite headcanon. It just makes all the sense and doesn’t detract anything from the canon story. It freaking adds to it.
Like, consider Harry’s hair. It gets mentioned, over and over again, to be a black goddamn mess and a nightmare to anyone who tries to manage it. That kind of hair texture? Virtually unknown to anglo-saxons. What could possibly cause that? White people trying to manage mixed-kid hair with no fucking idea what they’re doing.
The Dursleys acting all uptight and embarrassed whenever the mere implication of the sight of Harry would shame them, as if Everyone Could Tell he’s not, like, Dudley’s brother or something.
Green eyes? I’ve seen mixed people with green eyes before. For someone with one black parent and one white one it’s really not all that uncommon.
All in all a good headcanon doesn’t contradict the canon, but improves it. This one, all in all, does exactly that. It’s a perfect example.