Jay linked to the rather awesome Multicolr service that lets you select up to 10 colours using the colour-picker on the page [selecting a colour more than once to add some bias should you wish] and the Multicolr app goes away and does its thing. 'Its thing' was, back in July, a search of the Interesting photos that make it into Flickr's Explore pages.
How times have changed. Just three months later, the colour search has a 10 million strong database of Creative-Commons photos the Idée Inc. folks have indexed from Flickr using their own 'visual similarity technology' and Multicolr now presents you with a grid of 50 matching CC-licenced photos. And yes, you read it right: that's 10 million Creative Commons-licenced photos indexed.
After playing with this new version of the mashup I'm still astounded - both by how the mashup works now [with a far larger image pool that previously], and by potential further uses. Multicolr, with perhaps tag-refinement for helping you find just the right photo subject, would be image-search nirvana - no doubt Idée [and I daresay Flickr] must be aware of this.