I had no idea that there was experimental Fade to Black Polaroid film - in which the exposed image goes through a number of color shifts until, eventually, it becomes all black (the process can be stopped, its not just for taking photos that quickly disappear). In poking around for more information about the stuff I ran across this tutorial but it's unclear if that experimental film is required for the techniques explained to actually work. Either way, it's a great concept with some great applications.