I tried to recreate the recently appearing "spanish castle" illusion, which fundamentally seems no different from the usual "stare at this thing for a while and then at a white wall" thing, except that apparently contrary effect lasts much, much longer if you rely only on inverting hues. Here's my attempt using a photo ganked from techstep (hope that's okay, rob, by the way. Let me know if not and I'll use one of my own... I don't have anything quite as colorful :)