Technical Stuff

First of all some mention of my source material. There are some wonderful tutorials on the WWW and one of them is a concise set of instructions for taking your custom skins, eyes and hairs and making them default. I'd love to link you directly to it (but that's plain rude) so, go to The Bogsims Institute and look under Tutorials, it's not that hard to spot.

The way skins work in your game is really very simple. EA gave each of its default skins a number;
EA Light 0.10
EA Tan 0.30
EA Olive 0.60
EA Dark 0.90
So that if your mother Sim is EA Light (0.10) and your father Sims is EA Dark (0.90) your children Sims can have a skin darkness value of 0.10 up to and including 0.90. Which meant that they could be light, tan, olive or dark.
And if you add skins with darkness values on or between the EA ones our example Sim children might display any of those skins too.