idée inc. has a fantastic image-search tool that looks through over 10 million Creative Commons images on Flickr based on the color of your choosing. You simply click the color palette on the page to select up to 10 colors at a time and the site displays a wall of image thumbnails which use a predominant amount of the color(s) you chose. While clicking a single color yields impressive results, clicking a few more colors displays a new set of thumbnails (extremely quickly) with remarkable results! I find this site to be incredibly useful, and worthy of bookmarking.