[Hellmark]

If it was primarily white being sent to the front (IE, being made visible), that would be feasible, but the black gets brought to the front. It is easier to clear the screen by sending forward the black particles, and having them bring back their buddies in one mass. If they just fired the white particles to the front, well, the black ones that remained behind would still be there and thus you'd still see a ghost of what was there previously.

EDIT: I should clarify. What I said before may be interpreted another way. Both white and black get sent up front, but ghosting happens at the edge of the differing colors, and where the colors are mixed. The only real way is to invert the screen (White to black, black to white). Any particles on the edge, that get dragged along with those of the cell next door, get corrected then (much like I was saying before).