There is a generic urban-myth type story that every librarian has heard, about the patron who is looking for a book, and who knows nothing of the author or title but knows that the cover was a certain colour.
Well, in a library that used this type of classification system, no problem.
On the spine label I would suggest instead of the Dewey or LC number that we could use the Hex Colors system used for the web. So that for instance if I wanted a book in a particularly nice pink I would look under #FF 69 B4
BTW, did you know you can search Google images on a particular colour (see below) but as yet you cannot search Google books for colours, this no doubt will come.