Research shows that often people remember books by the color of its cover ("Well, it's red..."). For some reason, organizing books by their color has been in the news recently. Peterme showed a picture of his bookshelf organized by color in The Tension between the Personal and the Public, there was the Adobe Bookshop in San Francisco that agreed to allow its estimated 20,000 books to be reclassified by color. Cataloblog points to an older effort by the New England School of Law to provide color as a limit on searching. My contribution to the list: the international childrens digital library provides color as one of their facets in the interface.
The England School of Law interface.
The ICD interface includes color and shape as facets.
The Adobe bookshop organized by color.
Peter Merholz's bookshelf.
A google search for color indexing reveals more. Or try Google scholar for papers.