So fans were right, she does have green hair but it's a different tone from what they thought.
Also, if you told me this was the cover for a hentai doujin, I wouldn't even question it.

In Japan, nudity and sexuality (specially of the female kind) are in general more accepted in media then most other contries.
And usually in shonen mangas, as long as they don't actually show nipples or genitals, this level of nudity is fine.

And as a reminder, Shonen means "young boy", aka "adolescent boys".
And this is a series that has one of the female villain sometimes having to fight naked.