One thing of note, there is not a mandatory link between skin color and culture. Not in our world and definitely not in the Wheel of Time. Jordan often mixed a cultural history/inspiration from our world with different physical characteristics than are commonly associated with that culture. I believe he did that on purpose. So that the dwellers in the Waste had red hair but their cultural inspiration was influences by many of the various tribal cultures throughout the world (especially the Native Americans), so that the Descendants of King Arthur had Asian inspiration and some of which had dark skin.
Based on how Jordan wrote the series and how the Wheel works, there is no reason that you cannot have mild pigmentation increases in the Two Rivers and still have everyone have the same culture. They came from far away in a time of globalism and free travel. Then they settled down into an isolated area and kept to themselves. Just because there is a (honestly very small) amount of pigment variation does not mean that they cannot be realistically a single culture. We haven't seen a combination of Irish paleness with a deep rich Congo tone, we have seen people within a couple shades of each other.
The reason we pick up on the pigmentation differences is because our culture and history developed differently than theirs. So we need to stop looking at it through the lens of our personal American experience and consider how it might have worked in a different set of circumstances.
As mentioned, we can disagree about the casting choices, we can be surprised because it doesn't match our mental image of the characters, but I say we give this a shot and I hope that we will all be pleasantly surprised.