I disagree, I don’t think that’s what’s happening here. I think it represents her moving forward in her life and she’s going back to her natural hair color as a conscious decision of her own.
I don’t think she’s doing it for anyone or to be presentable (if so, she would have dyed it black for the piano rehearsal thing they did with the students), she’s just feeling like she’s re-attained her sense of self and purpose.
She had to have dyed it back though. For him to come home and be surprised it had to happen in one day while he was gone to work or whatever. Otherwise it would take
weeks to grow out to the point that all the blonde portions can be cut off and leave her with a complete amount of hair to style.
and I recognize what it is within the story, but often times it feels like, from an authorial standpoint, those decisions are made in deference to Japan's extreme tendency towards homogeneity and attachment to specific ideas of of what is right/proper/correct. Obviously she would've dyed it before the recital/lessons thing if it was meant for presentation in-universe, but that scene also had to go as it did so that we'd get all the judgy pricks watching her and expecting she'd suck/fail so that she could show them up in the end. But a lot of times when this sort of thing happens and a very gal/delinquent character drops their dyed hair or piercings or whatever it's presented like a sign of "ok, she's
normal now and that's better." in a way that makes it seem like wanting to be a bit different was the last vestiges of their renegade past.
Obviously this isn't a series meant to go into deep detail like that though. It was more like the scene just tripped something that made me remember past series (Which I couldn't even specifically name) where the same thing happened and it
was clear there was a message behind it.