Not badly reasoned, I admit, BUT the Northmen are caucasian and the Dornish are dark-skinned, it's just the way it goes, and that is part of their respective cultural identities . Same for the realms of the North/Nilfgaard/Skellige and ZerrikaniaThis is why I mentioned cultures specifically. If a person with dark skin is born in France, lives their life in France and is a part of French culture then that person is French. Back during the Medieval years most people in a given European country being "white" was incidental. Assuming it's even true or true to the degree we're assuming. The individual was not French, German, English or Welsh because of their skin pigmentation. They were French, German, English or Welsh because of their cultural behaviour.
Respecting the culture is respecting the culture. If the Elven character is depicted as being of Elven culture that condition has been satisfied. So most people were allegedly "white" in the historical regions the fantasy show is based upon at the time. Who cares? Toss an actor with any skin pigmentation imaginable into the role of an Elf. If they behave like an Elf and live and breath the culture they are an Elf.
It strikes me as a petty complaint.
That's not really true. It's the point trying to be made above. The Dornish are the Dornish because they exhibit the customs, behaviour and belief systems of the Dornish. That is what makes them Dorne. The North is the North because they behave as the North. It makes no difference which culture you pick. Whether it be a fantasy or historical one. The culture is the culture because of their behaviour. Not because of skin coloration.