Until the new feature propagates to all Google data centers, you can still use the image filter manually and even select two predominant colors.
Google Image Search has previously added filters for photos, images that contain faces, clip arts, line drawings and I expect to see even more options that rely on image analysis. The Neven Vision acquisition will certainly bring some interesting visible results. "Neven Vision comes to Google with deep technology and expertise around automatically extracting information from a photo. It could be as simple as detecting whether or not a photo contains a person, or, one day, as complex as recognizing people, places, and objects."
{ Thanks, Chris. }