What?? No.
Colin Powell being casted as General Patton does not remove anything from the other black generals, because the story is not about them, it is about General Patton, a person whose story had nothing to do with race. Again, Patton's story is not about race, it is about military leadership. A black actor can deliver that without an issue. Did you see the Patton movie? Did it involved the racial struggles of the pilots? No it did not, because the story is not about them.
Now, the narrative of the movie will fall apart if in the course of the movie, he would meet Benjamin Davis and congratulates him on being the first Black General. Then the movie would make no sense.
But since it doesn't, then it would be perfectly fine.
Also most of the fairy tales of old times are 100% fictional. They are not based on historical figures. Who was Cinderella based on, and all the princes and princesses of disney.
Only snow white because her name on the book is a direct reference of her color, but for cinderella/tinkerbell etc, race of the actor has very very little significance.