And share this with a graphic design colleague: From Idee labs, a tool that can search 10 million of the "most interesting" photographs on Flickr...by color. Click on the color spectrum map and choose up to 10 colors--or click one more than once to increase its prominence. All the photos have a Creative Commons license, and the search tool also works with a stock photography collection.
And speaking of licenses, the New York Times also offers this handy guide to using images that are in the public domain, to help you find eligible images and understand their copyright-free use. (Photo of the John F. Kennedy inaugural parade by Al Finn for LIFE)