Why did Nibutani Shinka changed her hair color?
It wasn't mentioned anywhere in the anime, I think. I just chalked that up to "trying to live a normal life, devoid of delusions and thus avoiding encounters with other Chuuni practitioners" by changing her image, like creating a new self - so to say. In addition to that she also arranged her hair in a more lady-like manner, adapted a long skirt and changed her normal black socks to white (gasp!), truly sporting an image change.



It sounds superstitious of Shinka and if you ask me, it was still actually a bit of a Chuuni thing, what she did. And it turns out I was mostly right, it says in the Wikia  that: 


  In the second season, Chuunibyou demo Koi ga Shitai! Ren, first episode, Shinka dyes her hair black as she doesn't want to be recognized by other students.


EDIT: Sorry about that, I didn't what I was writing. It wasn't explained in the anime, as far as I can recall (maybe in the LN, but they're stories are too different anyway). Again this is pure speculation in my part, but I guess after realizing her futile attempt at it and the fact that she's already attached to the other characters in one way or the other, she just changed it back.