Dark Norwegians?

I hope this doesn't sound terribly naive. I just got a picture of my great, great grandmother, Mary Ann Forlie, and it's not what I expected. She has dark, dark hair and eyes...Some of her kids in the picture have white hair and some of the others look dark, almost like Native Americans. She came across on the Emelie with her parents and I have several records, military and otherwise stating there were no other marriages for either she or her husband...So, the kids are theirs, only. Can someone shed some light on this for me?