They have nuances sure, but a character who asks for consent and has boundaries and a character who rapes and tortures are two completely different characters. The colors exist to separate these big differences. Of course characters who all do weird things will just be a nuance of red, but to help people avoid reading those nuances of red all together, it helps to have the label.
POTN, Jinx, Pearl Boy, Reverse Thinking, Waterside night, etc, these would all fall in the category of red. I haven't read any of them, idk their distinct nuances, but it doesn't matter, because people who don't like to read toxic stories should avoid all of these together.

Labelling characters as *color* flag is weird to me. Like reducing their value to green, yellow, red when they all have nuances? Some would compare this and that character as both red flags but they aren't remotely the same. "Oh they're a red flag but he still loves MC so he's a green flag!" "This character was such a green flag, then he turned yellow, then red, then green again."
I also don't get people not wanting to read a story because others labelled it as a red flag. It's not like everyone else's standards for what is considered *color* the same. Maybe I read too much but I went and read Painter of the Night half expecting him to be a monster but he didn't even exceed my expectations.
Just read the damn story and form an opinion.