Just stare at the dot for 30 seconds. Then, without moving your eyes, move the mouse over the image. It will look like it's in color until you move your eyes.It does too, it looks like a colour photograph, but snaps back into black and white as soon as you move your eyes, clearing the afterimage from your vision.
Even cooler, moving your eyes away doesn't completely clear the afterimage - if you move back to the spot, you'll see some residual blue in the sky.
holy crap. my eyes hurt now.
(and the URL field is apparently truncated??)