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This option was actually there before, but you had to manually add "&imgcolor=color" to the end of your query; now, it's much easier to access. Under advanced options, Google Image Search already has quite a number of interesting options, including filters for image content type, size, and format.
This new addition to Google Image Search is important because several visual search engines have built their products on precisely this functionality; retrievr, which we've mentioned in our odd Flickr search engines roundup, is one similar service. Now that Google offers this option, I don't see many users going elsewhere because of it.
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Image courtesy of Google Operating System