See through ePaint that takes on the colours of the image projected onto it
Street ads that just need someone to project an image on to it for a second, click save and the pigment in the see-through paint changes colour, would save heaps of time for bill stickers who want to switch ads quickly. Not sure if its been invented yet though.
nice idea, especially if it looks like chameleon paint if nobody is advertising on it at the mo. or it might just look white or dirty, in a city.