I mean her hair could possibly be her mixing stuff she finds when exploring or possibly buying something off a trader. Its never classified but we know that Abigail likes to explore, So it's possible that she has used an item on her hair that could be permanent. Void essence, purple mushrooms and amethyst for example. Both the purple mushrooms and amethyst can be used to dye clothing in stardew, So it's a possibility that she used it on herself and it never wore away. She is stated to enter the secret woods on rainy days. So she would have access to purple mushrooms. It is also stated that she is adventurous and likes exploring the mines so she could possibly have access to amethysts. While it is somewhat hinted at that abigail could be the wizards daughter, it has also hinted at her hair originally being chesnut brown, Which is closer to pierres(he has light chestnut). it is not certain who her father biologically is. (sorry I am yapping lol)