I think my sister suggested that perhaps they had a very large gene pool themselves. Somehow, God gave their DNA all the capacity for skin types and hair color and eye shapes one could have.
I imagine this to make sense; I got bad grades in Biology. I picture their fifth child coming out with a head of red hair, and they exclaiming, "Oh, look, humans can have red hair too!"
And then, when Eve was pregnant again, "I wonder if this one might have blue hair?"
I doubt either of them asked, like Mark, "Will the baby be human?" Though I do wonder how Eve had any idea what was happening to her. I guess God did a lot of talking with them in the Garden before the Fall.
I hope we get to meet them in heaven.