If you have CC hair, some CC hairs don't use the game's default hair colors either, which makes them just totally blow the concept of hair color genetics out the window.
Genetics in TS4 aren't all that well developed compared to past games in the series either and you can get totally random results unrelated to the family tree at all, but the most shocking variances I've experienced in hair color have usually been because either one of the parents had CC hair with non-default hair color options or the child itself rolled a CC hair with non-default hair color options.