Responses
If it's by the spoiler ... It's book 2, around page 250
It's this one , page 30 on the spoiler forum https://forum.novelupdates.com/threads/when-the-villainess-loves-%EC%95%85%EB%85%80%EA%B0%80-%EC%82%AC%EB%9E%91%ED%95%A0-%EB%95%8C.112441/page-30#post-6215342
Before this chapter I can't imagine Tia's hair in a darker shade. But now, I can't wait until she truly becomes Litia Unnie's twin sister (≧∀≦) so pretty!
* If I'm not mistaken (from novelupdates spoiler forum), the MC is actually Litia Unnie's twin sister... So yeah! Once she assimilate both body and soul, her hair will change colour (if I'm not mistaken) *
Here's the link : https://forum.novelupdates.com/threads/when-the-villainess-loves-%EC%95%85%EB%85%80%EA%B0%80-%EC%82%AC%EB%9E%91%ED%95%A0-%EB%95%8C.112441/