Messing around with different aesthetics and groups for this story, and I'm wondering how to go about this. I wanted to do a setting where a region is full of basically ancient Egyptian inspired cultures but the climate is colder instead of a hot arid desert. I've seen it speculated that certain groups were able to maintain their darker pigment even in colder climates if they kept a diet of vitamin D and were still in a region exposed to sunlight in a place with high latitude (correct me if these aren't entirely accurate).
Now I'm working on a group in the same world that are basically desert "Vikings". most of the suggestions I've been given end up making them basically Middle Eastern ethnic groups, but I wanted to keep their ethnic background and phenotype Scandinavian-inspired.
Basically for these two groups I want to keep the selective pressures that cause pigment changes. Would the opposite of the vitamin D thing be if their culture ate a lot of folate? I figured since in a hot and exposed place UV destroys folate stores, they would have to have folate diet that would make up for this. Would they have to be recent migrants to the region for it to be plausible, or can it still be plausible for them to be natives to this hot desert region? Can the elements that create this scenario be realistic, or do they need to be more fantastical? like say, fantasy foods or circumstances that would cause this.