George L. sent me a nice example of a website integrating image search features: Become.com offers a search ability for color based retrieval of clothing. Just use the “find by color” feature in the right hand side column at the top and you will be impressed how much fun image retrieval can be :).
One can also observe how inaccurate algorithms get when the background is integrated in the color histogram (e.g. with this search parameters, where I searched for grey and found also some blue images – or even in the presented screenshot where an obviously not red dress is shown before a red background). However this is a still unsolved problem: How to separate background and foreground in still images?