By the way, both Jacob and his missus are ever such a light shade of brown - so I'm going back to my original theory about high status Africans being lighter-skinned than the masses.
I did some work with a few community workers from Sudan for a forum against racism (here in Australia, not in Sudan) and the bloke said that was a major cause of separation in Sudan, even amongst people native to there: as people were progressively darker, the worse they were regarded socially, and the more negatively they were descriminated against.
I don't really have any way to back this up aside from an anecdote, though.
But what I'm trying to say is, your observations are probably correct.