Finding images with pure colours
How about doing this?

Blur the image using some fast blurring algorithm. (Search for stack blur or box blur)
Compute standard deviation of the pixels in RGB domain, once for each color.
Discard the image if the standard deviation is beyond a certain threshold.
I've read a number of questions on finding the colour palette of an image, but my problem is slightly different. I'm looking for images made up of pure colours: pictures of the open sky, colourful photo backgrounds, red brick walls etc.
So far I've used the App Engine Image.histogram() function to produce a histogram, filter out values below a certain occurrence threshold, and average the remaining ones down. That still seems to leave in a lot of extraneous photographs where there are blobs of pure colour in a mixed bag of other photos.
Any ideas much appreciated!