I'd guess that the main sequence has to do with melanin...
S1 = light
S2 = tan
S3 = medium
S4 = dark
... but are there also genetic skin tones for aliens, zombies, and whatevers?
I'd also guess that if you bin all the Warlokk skin tone as an S1, S2, S3, or S4 the kids might have any body shape with the same melanin density. That would mean that binning the body shapes is a bad idea if you want offspring to inherit the body shape.