I'm helping a Dear Friend move and upgrade her blogs, and now we're working on colors. I want to present her with images to accent the color theme. I had heard that there color-based image search engines, and a quick Google search uncovered Yotophoto. [2009. Dead, it seems.]
You can search by key terms, licensing (I like Creative Commons and GPL liberal licensing; public domain images, too) or particular sites. I think we should find some goodies.
Here are some images that coordinate with this blog's bordering green and the nameplate's purple. (I'm rather partial to the swirly purple NASA nebula.)
[2009. Yotophoto is toast.]