Did anyone wonder that even though Arda was mostly populated by European looking people since it is based on Celtic and Northern European culture, is it still possible that races like elves and dwarves can still have different skin tone or background which differs from the main population? Probably not many of them, but they still exist.
Firstly for the elves, it's true that most of them have pale skin due to their specific origins, like the Sindarian and Noldor and those that dwell in Valinor. But what about the Avari? I always think they are a special case since they never enter Valinor nor Beleriand (only very few of them). Instead, they dwell in the East, have their own tribes and lifestyles and wander all across the wild lands of Middle earth. And they're probably considered more tribal and wilder than the Noldor and even Sindarian. Since they dwell in the East, which has distinctive climate and geography, is it possible that their appearances and physique would alter due to the harsher environment they live?
As for the dwarves, since they are divided into 7 houses and scattered in different corners in Arda, I suppose it is possible that they might have descendants which has different skin tones and cultures too. They may almost look identical, but we can interpret this as looking "identical" amongst their race.
Look, to anyone to watch ROP, I'm not saying that it's fine for them to randomly cast the characters without considering Tolkien's lore and cultural elements he placed in the story. But if they really want to do it, they should have added a reasonable background for each of these character. Eg. It's more logical to say thag Arodir is an Avari from the east instead of from Beleriand, and Dis should be from a dwarf house which has black descendants (Amazon had chances to dive in since these parts are left unexplained sometimes). Just like how the Harad look different from the men of Gondor and Rohan because of their cultural and geographical background too.
