They're not actually looking for colouring books, it's a language learning tool that breaks the kanji down with kana shown above them to aid in learning unfamiliar kanji, like this.
Having said that, asking for hentai specifically is goofy as shit. Just be normal-ish and ask for manga that isn't for little kids. I'd understand if they wanted to read Mob Psycho 100 with furigana, rather than Doraemon.