I always thought that DNA was random anyway. In the end we're all derived from fish, right? So six fingers is always an option, regardless of how unlikely. Just the chance of your little swimmers having a 6 finger or even a mutated and totally different chromosome being highly unlikely. Same would be true for skin pigment, just that the highest survival rate for those living in low UV radiation to better absorb vitamin D, where they would tend to suffer from cancer in higher sunlight conditions. Strange that such a small thing in a modern society where those things mean so much less to us with vitamin supplements and sunscreen can solve these problems.
Do I have a point? It's that in a way, we're all mixed, and genetic diversity is a good thing, think of hemophilia. Trying to keep it separated by skin pigment is about the lamest excuse there is.