This is exactly what Brandon Sanderson said when it comes to any potential adaption. He said Asian would probably not be the best pick for all characters. He said Shallan should be Caucassian not because she is written as such, but because this is how most readers are defaulting towards seeing her due to the red-headed girl carrying strong imagery. When I asked him about the blond-headed, blue-eyed, imagery Adolin had going on, Brandon answered the same: the casting could go Caucassian, that would be good enough even if this wouldn't be his personal pick. He will not fight for his personal pick and he is willing to give casting directors latitude here.

I dunno why some readers get angry or absolutely want to stick with an Asian artificially dyed in blond actor for Adolin when closer to his book description options are available. I mean, everyone else will be Asian... So what if the red-headed and the blond-headed actors are actually cast as such? I dunno why this always turns out so sour. It'd be fun if we could discuss actors without this being an issue. The author said it was fine. Let's take his words for it.