I asked this over at MTS, but I'm not sure I'll get an answer since it seems to have been buried. Then I thought, "Maybe the wise folks on LJ would know!"
I'm not sure I can explain properly, but here goes... In my game, I had a Sim named Bertram who was an alien: green skin, blue eyes. (Half-alien really, but I think that was lost when I had to rebuild -- I gave him alien skin and blue eyes, but no parents.)
Bertram had a son (Albert) with an S3 dormie. Albert displays an S3 skintone.
Albert married Buttercup, who displays S2. (Her parents were S1 and S3.)
Albert and Buttercup have three children. The youngest, Skye, displays S1 -- but the inside of her mouth is blue: blue gums, blue cheeks, blue inner lips. Does this mean she is somehow carrying the alien skintone as a recessive? Or is she glitched somehow?
EDIT: Here is a picture:
I have not been able to get a look at Albert's mouth to see if he has the blue gums thing going on. EDIT 2: He doesn't.