Observations and results
Did you see a ghost or shape when you looked at the blank sheet of paper? Did you notice that colors inverted to their complement—red became green, blue turned orange, etcetera?
Illusions can be spooky—especially if you have not learned the science behind them. But they also reveal how your eyes—and brain—work together to process visual information. When you use just one eye at a time, you probably noticed that only the eye that had been staring could see the ghostly illusion. You may have also observed that the ghostly image appeared in different colors. Why was that? The image you saw on the blank paper is called an afterimage. When you stare at an image for a long period of time, the cones in your eye become overstimulated and lose sensitivity. So when you're looking at, for example, a red ghost, your red-sensitive cones become very active and eventually—if you don't blink—exhausted. When you then look at the blank page, your eyes are still receiving many different color signals. But instead of your cones sending equal signals and balancing out to perceive a white page, your tired red cones don't send a signal whereas the blue and green cones do, leaving you with an aqua-green afterimage.
More to explore
The Neuroscience of Yorick's Ghost and Other Afterimages from Scientific American
The Ghost Hand Illusion from Scientific American MIND
Sight (Vision) activities from Eric H. Chudler, Neuroscience for Kids
Are Your Eyes Playing Tricks on You? from Science Buddies
Color Aftereffect from IllusionWorks