Not trying to get to the biological area (like that light-brown couple who had twins, one black and one white), so let's think Sims.
Your green sim is a strong green color (it's to the right-most part of the skin color slider), and the woman is a light color. The baby can only go through one selective skin color, so it picked the mother's, but it got the father's "darkness" (the right-part-of-the-slider thing).