How did Triton and Athena have blonde and black-haired children?

You have to keep in mind that genetics come from all previous generations. While its true that Athena and Triton don't have brown, black, or blonde hair that doesn't mean that they don't carry the gene for it. For Ariel's vibrant red colored hair I'm just gonna say she isn't human so maybe mermaids can have different hair colors than humans.

Maybe because Triton's parents, and Athena's parents had blonde or black hair. (the mom had black hair, the dad had blonde- or vice-versa.)

There are plenty of parents who have children with different color hair from them...Me and my husband have brown hair and our oldest 2 daughters have brown hair, but our youngest has red hair...maybe Triton and Athena have people in thier families with Blond,Brown,and Black hair...I have neices with Blond hair when most of the family has Brown...it happens....I have even heard of 2 white people having a black child (not because of cheating) because there was a black person somewhere down the family line (that is rare though)We are not exactly sure still where my daughter got her red hair...it is very odd...

Well, I have a friend who has super Ginger hair and neither of his parents have it. If you have parents with both blonde hair, it's possible to get black hair as long as it's somewhere in your gene pool (grandparents, aunts, uncles). So maybe some of their relatives had black and blonde hair!

I have a friend and both her parents have brown hair. She has blonde hair and both her older brother and younger sister have bright red hair.