I don't approve of this. He doesn't own the land. He's not even a citizen of Iceland. He didn't get a review by a local ranger or committee. He's an obnoxious tourist.
He could color a fountain, this is often done in my town when the baseball Cardinals have a division or league championship. He could color a small waterfall by submerging some gelled marine flashlights (and then retrieving them after he is done). He could backlight the geyser at night, gelling his lights.