Could have been my starter slave on a 'quick-start' playthrough. Sounds dumb, but I have so many random saves made to experiment, I tend to forget who was the starter (Before "Grateful" came in too). I know quickstart breaks certain other rules, being able to have a Dryad starter in story mode etc.
Gonna test some more. Hoping you're right, as the less skin tones available, the simpler it is to cover all probable eventualities in a portrait pack.
Cheers Mcgee
EDIT: mb. You were completely right. After assaulting 42 elves/faries, not one had brown or dark skin. It's only starter slaves, that are human or don't have stated ranges, which can be given those hues (mainmenu.gd).