Google is constantly updating their products. Most of these updates come via their "Labs" where they test out crazy and sometimes even crazier ideas for possible features and updates to their products.
Google Image Search somewhat recently added the ability to search by color. Which I think is fantastic! Here is a search for moth, constrained to only those in the yellow color space. What’s more is that Google Labs is testing "Similar images", which I think will also be a great update.