Isaac Newton claimed there were seven colours in the rainbow. Personally I count six – there’s barely room for blue and purple, let alone indigo and violet, and Newton only went for seven because it’s a nice mystical number. But the association’s there.

I long felt that our catalogues should allow users to search by the colour of a book. After all, that’s one of the major pieces of metadata they remember. “Hi, I want to borrow the green thermodynamics book,” they’ll say. I’ve never quite understood why we painstakingly catalogue ix, 165 p. : ill. ; 24 cm. and don’t add #0033cc to the end. (Or, for that matter, why size and number of pages and presence/absence of illustrations aren’t searchable fields: it’d be handy to be able to search for a book on geraniums with more than 200 pages and col. ill.)

Some years back I saw a prototype of a library catalogue that did allow searching by colour; if I recall correctly, it took the cover images from [some source] and averaged the hex values. I don’t recall whether it let you search by picking a spot on a colour wheel or if that part was just my invention and it only let you choose from a list of colours.

In any case, this never got picked up on. At the time I saw two reasons, but I think now there are three:

• Technology hadn’t advanced far enough: That is, while it was technically possible it wasn’t technically easy. Most libraries at the time didn’t have cover images in their catalogues. Vast amounts of metadata would have had to be added, and custom code would have had to be integrated into at least the public search interface of the library management system.
• The customer isn’t always right: As often as not, “the orange risk management book” turns out to be black. Even when new editions and rebinding battered copies isn’t in play.
• Technology has advanced too far: Who on earth is going to remember what colour their ebook cover is? Well, some and sometimes, but colour is a much less pertinent detail in the electronic context. And there are a lot of other ways to search now too, from all those “refine” options to full-text searching (a boon for all those “I photocopied this page and now I can’t remember where it’s from” questions).

I think it’s interesting how technological changes make some things possible and others redundant.