If we went to Croatia and Serbia both wouldn't concern themselves white or they wouldn't consider the other as white? What would they call themselves?

I think they would consider themselves white from a different nations. Many north american natives come from different tribes but they are still Indian. Same thing.

They'd consider themselves Croatian and Serbian. Whiteness is an American construct that is absolutely meaningless and smushes literal millenia of history into OBE category because your country's history is so short.

Please refrain from spreading nonsense you clearly know nothing about.