I haven't adopted that many, but the few that I did varied of skin tones and could be similar or widely different from the adoptive parents.
The thing that gets me is the skinny little girl I adopted one day, aged to child and was still skinny and then when she became teen she was quite big. And I had to laugh at her traits, party animal, athletic and loves to ride horses. This was when I had pets.
it never seems to fail that when I adopt one and the child or toddler is skinny as a rail and then age up to being quite over weight. It isn't the weight that gets to me, it is the extreme of the differences. A child who would wear a size small ages to teen and goes to 2X.
Skin tones vary even in families of natural born children, my sim just had a baby and she and hubby have fair skin. But he had dark skin. His grandfather had a dark skin so I guess it would be genetic, but it looked odd to have a blond haired fair father with a dark haired fair skinned mom have a kid with dark skin and reddish hair.
You just have to love (or not
) the wild things that the game can toss in without expecting it.