it has to do with the things on your back of your eye(don't know the english terms), there are different receptors for different colors, and there is a chemical, maybe an enzyme, that gets recycled over and over. if you stare at one color, all those receptors for that color at that place are out of function for a wile untill that chemical is recycled back again, so if you look to a white surface you see all colors but that one, so the negative.