I found thecolorof.com this evening (via Curiosity Counts), which layers recent images from Flickr sharing a common tag over each other to create swatches that resemble abstract impressionist paintings. Naturally, I spent a bit of time trying out various cities. Many wind up very similar--red/gold on the bottom, bluish on the top--which I'm guessing has something to do with the preponderance of skyline photos. But the results for some are spot-on, and even downright gorgeous. A few favorites:
What color is your city?