From what single picture I could find of a Nefantar they seem to be Caucasian looking. The character in question, Kasiha Valkanir, looks similar to female Civire. Maybe more Mediterranean.
Skarkmanir are described as looking like slighter built Brutor with darker skin. I imagine them looking like the Afar people.
Krusual are described as “tall and brown skinned” which is very vague.
What about Thukkers?