Solved: Children's fantasy with rainbow-haired(?) people in a hidden valley

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Children's fantasy with rainbow-haired(?) people in a hidden valley

I think I read the book in the mid-90s, and I think the main character was an ordinary boy who somehow found a way to a valley where people with rainbow-colored hair lived. It might've been that their hair looked ordinary when they were outside their valley. He met a girl who lived there or followed her there or something.

There was probably some tunnel or moving rock or something through which you could get to the valley.

It was translated at least to Finnish.

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Found it, it was a Finnish book apparently, no wonder it was hard to find any info on it in English.