Every few minutes we do a search on flickr for new photos tagged "favcol". This is done using the flickr api - a way for other sites to make use of flickr's data.
We then analyse each photo to work out the average color, shown as a thick border around the photo:
You can see how many photos there are of different colors at the top of the page - the taller the bar, the more photos there have been.
All the photos are then averaged together, but colors can cancel each other out: very red pictures don't have much blue and very blue pictures don't have much red, so on average there's not much blue and not much red, which can mean green is left over.