I don't know what to say, I checked again the whole code and only dark elves are supposed to be born with brown/dark complexion (see Ivran, for instance).
There's a function, getrandomskincolor(), that returns a random color chosen among ['pale', 'fair', 'olive', 'tan', 'brown', 'dark', 'blue', 'purple', 'pale blue', 'green','jelly','teal'], but it's invoked only while altering the slave's skin in the lab or through a "mutate" spell (cast by the MC or caused by an excess of raw magic).
Eta: "It seems odd to me that humans don't spawn with dark or brown complexions." I totally agree.