This reminds me of an epigenetics class I took a few years back. It was quite technical and complicated, but I did my best to understand it. The X and Y chromosomes are genes that determine our sex. I learned that during the time the baby is in the womb, there are certain opportunities, based on the environment, that determine if that person will become a boy, or a girl, and their phenotype (hair color, eye color), etc. The X and Y genes get randomly inactivated, but sometimes not totally. Since we get XX from our mother and XY from our father, we can only use two of these four chromosomes, so the other two get inactivated. The XX genes determine if one is female, and XY determines if you are a male. Sometimes, if genes are not totally inactivated, they may show up elsewhere on a person's body, like a mustache on a woman. We all carry bits and pieces of inactivated genes left over from the time in the womb. So this girl may have gotten genes from both her parents and this showed up in her hair color.
Also, think of a calico cat. It has a mixture of colors, not just one color. These colors are due to epigenetics, where the genes are turned on or inactivated, based on the environment. Same with people. Here's a link that explains it a little:
https://pencilflip.medium.com/why-are-cats-calico-5a42c7171c6
I may be wrong, but these thoughts occurred to me when I read about the girl's hair color. Hope this helped explain it a bit.
