I'm going to throw a guess but I think she did it because she thinks the human race have better look or while thinking of being beautiful she wanted to somewhat look like the ml. It could be because they are both in master-servant contract, so if she changes she'll look somewhat like her master.
It because she feel insecure with her sisters, it was mention in the chapter that she is the weakest and maybe the black sheep among the sister, also with the curse that cast upon her make her more insecure with her appearance,. thats why she wish to be pretty and change her appearance when the guy give him the evolve stone..
I recommend the webtoon "METAMORPHOSIS" where there are beautiful green skinned demons
Okayyy but why did she have to change her skin's colour? That's just sad