TinEye Lab's Multicolr Search Engine is still the undisputed king. Search Flickr's vast image base by choosing up to 5 seperate colors, and narrow your search even further by using text tags.
Google Image Search
A powerful yet often overlooked feature of Google Image Search is the ability to narrow your results by color. In the nav bar click on search tools and use the color dropdown to filter your images.
Already a great resource for art, web and graphic inspiration and ideas, Designspiration also boasts a color search feature that lets you query its image base using up to 5 colors. Simply awesome.
Searching Shutterstocks immense database of images using their free tool Shutterstock Spectrum can produce some beautiful results. Even if you're not inclined to purchase anything, still give it a try.