The Multicolr Search Lab tool created by Idée Inc. allows to find images that share specified colours. There are others who offer this option.
Google Image Search lets you restrict the results based on color. For now, the option is not available in the user interface, but you can tweak the search results URL to try it. More informations are available at the Google Operating System Blog which is not affiliated with Google.
The Imagekind artshop lets you choose artworks by specifying a dominant and four other colors.
Etsy, an online shop of handmade products, has developed color search for their own product database.
PicItUp is a visual image search engine that combines word-based with color based search.
A COLOURlovers community, made up of people from around the world and from all walks of life, are gathering together under the common love of color.
Matt Mueller based the final project for his Computational Photography class on the Idée Labs tool Piximilar.
A server version (isk-daemon) and a desktop version (imgSeek) of an image database management system have been developed by Ricardo Niederberger Cabral as open-source project (licensed under the GPL). Color analysis is also a topic in the ImageMagick discussion forum.