Stare at the dot for thirty seconds, then move the mouse over the image and keep looking at the dot. The castle appears in colour until you move your eyes - neat trick!
Very cool, that's a phenomenon called of "ghost colors" (transl. from italian), similar to that one when you look for a while to a lit lamp bulb then you watch in a different direction and you see a dark ball. But this is very cool and well made. Thanks