This is a train wreck on all sorts of genetic levels.
Why even use hatred of their skin tone as a premise if the Fl isn't going to have that same skin tone or close to it in the main story? It makes zero sense to go here is the Fl who is hated because she's related to the dark skin people except she looks exactly like the oppressors but trust us she's black on the inside.
For their skin and hair to lighten to that extent there would need to be a lot of race mixing going on during those 300 years to the point no one is more than 1/8. That means the red hair had to come from one of the accepted light races and is not the result of 1st generation hair mixing genetics, its been 300 years so that hair is definitely from one or the other race and it can't be the dark skinned race because that red hair isn't dark. So, If their skin has basically become the same white tone and they have the hair color of accepted people there nothing to differentiate those people. How would hey know who to discriminate against since they also roam all over the place now, even more race mixing. You're not going to be able to tell who has "Dirty blood' ironically they can't either judging by that name.
Honestly the hair cant matter too much now or be a distinguishing feature since the king has black hair and some of the gypsies have blonde and blue hair? So it's just about skintone which is all white now so... what are we hating again?
But, three hundred years that's like 3- 4 generations? and even if every mixed kid only had a child with a lighter person there'd be some features from the other race mixed in sometimes, cant tell in this art but everyone looks the same anyways.