And here I like white-on-black because it reduces eyestrain if you’re working at a terminal (or terminal emulator) for long periods.
OK, on to the question. Afterimage is produced when you overstrain the cells on the retina that percieve light to the point that they keep sending impulses down the optic nerve after the stimulus is gone. Of course, the effect is greatest when there is great contrast in what you were looking at, especially in brightness. So with black-on-white, you are looking at an image that is mostly of the same intensity with some little black spots. With white-on-black, you’re looking at an image that’s mostly black with little points of brightness that overstimulate your little rods and cones on the back of your eyeball. Solution? Take breaks. Look out a window. Pick up a copy of Playboy/girl and … uh … ‘read’ that every so often.