again, there's a grey area in that "actors pretend to be other people for a living" so you should be able to have any actor play any role. except that's not true. you can't have a white dude play MLKjr, you can't have a jew play hitler, you can't have an asian female play george washington. even if you write a story in which race plays ZERO PART in those stories, the suspension of disbelief is too high.
even if you are making a story about mythology. let's take norse mythology and make all the gods black. a bunch of blonde hair blue eye swedes worshipped god that look nothing like them? most of them probably never even saw a black person. but so what, it's fictional, right?
okay, let's take some native american mythology and cast some white people and see how far we get.
either anyone can play anyone, or you have to be selective. you can't have it both ways.