Animation maybe..

Another idea instead of the colour thing, have a site where people can all add frames to an animation. The frames would be drawn in and all people would have to do is add colours like paint by numbers, and add different elements to the frames, sort of the same set up as the hugocreate website with its poster generator.
Would have to have a limited palette for each number so that the animation isn't too hard to look at and actually looks effective. but maybe not because who really cares... would have a little character in the animation that you could choose facial expressions for etc, and textures as well as colours so the image isn't too flat.