Originally posted by: Chronoshock
Seems like its just exhausting your red and blue (red+blue=magenta, blue+green=cyan, green+red=yellow) color receptors thus making a neutral image shaded green. Since you're staring at one spot, obviously that one particular area of receptors will be saturated, whereas if you follow the disappearing dot with your eyes, your field of vision constantly changes and there aren't magenta dots in any particular part of your vision for an extended amount of time.