We know 100% that Lily has pale skin. James can be poc cuz the text never says anything. And poc can be aggressive and act badly. Imo fans made him poc to sanitize him. "He's poc so he can't be violent that's racist!" It's their shield to always portray him as good. When I see white James in fics he retains a lot of his canon flaws but poc James fans turn him into a sunshine.
We know both of them are 100% white because if they weren't, it would have been mentioned, like how it's mentioned that Dumbledore's mother, Kendra, looks Native American, even though she's irrelevant and mentioned only once.
I know why fans choose James to change, and I think it's stupid. I'm a poc myself, I know from firsthand experience that other poc can be bad, so I have no issue with that. It's more so for the reasons you've already stated, that being fans thinking it erases James' bad behavior and makes him more sympathetic. For a fandom that wants representation because it's lacking, they sure do love choosing the worst ones to represent them and others (don't even get me started on those who say Barty Crouch Jr. is for "the gays," as if he isn't a massive creep who groomed kids and tried to murder one).
But I prefer him white bc it fits his highly privileged and careless energy. Idk if poc are oppressed by wizards but James feels like a typical highschool white boy who does whatever he wants bc he has the arrogance and audacity that society accepts in white boys. But regardless, if ppl want Harry to be mixed then James fits more bc Lily already has canon skin color, plus has red hair and green eyes.
I get that privilege and "careless energy" are usually associated with white people of that time, but the wizarding world isn't exactly the same as the Muggle world. They don't deal with race the way Muggles do, they are more so all about blood, family, and class. I mean, wasn't it literally said that they had a Black woman as a minister, or in power or something, long ago before Muggle women even had rights or something like that? We already know that there are wealthy, pureblooded, Black/poc wizards. That includes the Shacklebolts, the Zabinis, and there's probably more.
So, I don't really think it's impossible or implausible for James to be a poc, but I hate the reasons for it, and I think it makes more sense for Lily, given her storyline and everything. Anyway, in a case like this, where they're both most definitely white in canon, canon doesn't really mean anything. And given that the author herself doesn't seem to mind these types of changes, it's a total freebie. And btw, a poc can also have red hair, green eyes, and pale skin. Most of my family has darker skin tones and hair, but there are a couple scattered around with lighter skin and reddish hair, though their eye colors vary. It's not impossible!! (tho probably rare-ish?)