Why is their skin a different color?

Another question that one of my children has asked, thankfully in private and not in front of the family, is why one of their friends had parents who were a different color.  Instead of trying to define race, I went to a mirror with my child.  I showed my child my own skin color and we compared it to hers.  We compared our eyes and hair too and my child left happily chattering that most parents are different colors than their kids.