In Sanderson's most recent lecture (50:25 in) to his BYU Writing Class, he mentions that Alethkar natives resemble Asians. This came as a bit of a surprise to me, especially since I always imagined the Shin as the "Asians" of that world.
It's a little more complicated than I might have made it seem. Alethkar natives other than the Shin have the epicanthic fold, but the Alethi wouldn't look strictly Asian to you--they'd look like a race that you can't define, as we don't have them on earth. I use half-Asian/half-arab or half-asian/half-Polynesian models as my guide some of the time, but Alethi are going to have a tanner skin than some of those.
Some Horneaters might look Caucasian to you--but then, most will not. They'll seem like something alien, and not all of them have light skin; they tend to walk a spectrum between pale and coppery. Reshi and Herdazians will look closest to something like an indigenous Bolivian.
Shin would look the closest to Caucasian to you, but again, they're not an Earth ethnicity. So you might not be able to place them either.
A lot of the fanart has done a good job with this, and if you search through it, it might help you get an idea.