I always envisioned each planet / great house / ethnic group to be ethnically homogeneous, although different from each other.

I don't think it's very likely there would be some black Fremen and some mediterranean looking ones in the same sietch. Even if the planet was colonized with a diverse population, it would have been blended out by the time the events Dune take place.

But also it's soft sci fi so tons of things already aren't realistic. Ethnicities are a weird one to get hung up on.