I thought you had some ost recently discussing hair colors but I can't seem to find it now Just knew that I had some helpful info.
Neither of those are the post I read? It was something about how a kid ends up with a hair color unlike either parent (like the Desmond baby in that JLU comic). Fortunately, my info answers not only that question, but also the posts you'd linked. 😀
Both my parents and my brother had brunette hair, but I was born blonde. However my dad's sister was blonde (I don't have any color photos of when my paternal grandparents were young, so can't trace any further back. And her son came out platinum blond.
As for me, my blonde hair matched my maternal cousin's. Her mother was strawberry blonde - which my own hair darkened to as a teenager. But my hair also had brown mixed in, and even a few brunette strands. And now I'm ash blonde.
tl;dr - Hartley's going from blond to redhead is less complicated than the ways RL hair can change. 😉