Genetics pertaining to blue and green skin

So one of my sims is blue and she had a baby with a sim with a light skin color and their baby cave out white also. But one of my friends has a green sim and has green kids but the kids have one green parent and one white parent also. I was wondering if the skin color of your kids is just random when it comes to blue and green skin or if it was possible for my friend to be able to change the skin color of her kids. thanks