This one might especially interest Lily (whose colour-coordinated window displays are a popular and regularly-changing feature of the shop). At COLORlovers, there's a very nice blog post, Organizing bookshelves by color, on just this subject. It cites the Rainbow of Books Flickr group, which has further pretty examples. Perhaps the most ambitious example, though, was in 2004 when the artist Chris Cobb persuaded Adobe Bookshop, San Francisco, to let him reclassify its used book section (some 20,000 books) on this basis as a public installation called "There Is Nothing Wrong In This Whole Wide World". Here are some pictures.