I'm sure that several fans have wondered this over the years, and actually there are several ways for this to have happened within the possibilities allowed by Mendelian inheritance. Let's just mention, to get this out of the way, that both Reggie and her mother being purple puts forth an argument for purple hair being a dominant gene passed matrilineally within that bloodline: any daughter Reggie bears has a good chance of being purple as well--and any son as well.
By the same token, Otto being auburn absolutely requires at least one of either Ray's and/or Danni's ancestors to have been auburn. There are, after all, several members of their extended family we have never seen.
In fact, any of the couple's children could have been purple, or blond, or auburn, or of any other hair color accounted for in their respective familial roots.
Biology is confusing, isn't it? Well, that's what makes it so interesting.