A terrific new tool for searching by color is the Multicolr Search Lab by the folks at idée.
The idea is simple enough - choose any color or combination of colors you desire and the search engine will search through more than 10,000,000 Creative Commons images on Flickr to find images that match your color choices. In the sample below, you can see I chose the combination of a medium blue, pale yellow, and pale sorta peachy color.
The color palette on the right allows you to select your colors just by clicking. You can click the same color several times to increase its prominence. Down below, you can remove some of your color choices just by clicking the little trash can.
This is a great tool for finding images that match a color scheme or for working on activities around a theme.
While visiting the idée site, don't miss out on their other search engines. The Visual Search lab allows you to choose one photo from a collection of nearly 3 million photos, then find other photos that are visually similar. There is also the BYO Image Search lab. In this engine, you upload one of your own photos and it will search for a visually similar photo.
All of these are nice tools for quickly finding photos that "match" according to color or other visually similar details.