When I was in Tokyo I saw a mother on the subway with candy red hair. There were others but she was the most notable. Also two of the people I met had bleached their hair brown. All were adults. If anything I felt hair color was less taboo in Tokyo than where I came from. It seems odd that they would have regressed since then so I’m wondering where you got your info.

Coloring your hair as a student can get you in deep shit. Someone invented a temporary black hair dye for this, which I found bizarre until this was explained to me.