Impractical? Depends on the readers and the context. I remember my books, especially my art reference books, by colour and typography and “the right place”. As librarians know too well, people think they know where to reshelve books, but they’re often wrong. I’m looking forward to rainbow order in my new place. Rookies will still make serendipitous discoveries, have a better chance of reshelving correctly by colour, and if they don’t, the stray books will stand out to me like flashing neon. YMMV.