What happens when I click "Make this a color by number book"?

The book becomes a Color by Number book: every page is colored from a palette of named colors and numbered, and the card then holds the color by number settings. The top bar then says "Activity Studio · Color by Number". There is no button to turn it back into a plain coloring book. From then on the book belongs to the Color by Number studio: it opens only while you own that studio, and its exports count toward that studio's guarantee, not Coloring Books'. The Color by Number knowledgebase explains its settings.