Watch the video above. Don't skip ahead. Follow the simple instructions, given by author and psychologist Richard Wiseman, and after about a minute, you will experience something pretty wild: The black-and-white image will, ever so briefly, appear in full color.
How this bizarre illusion works
The secret here has to do with a phenomenon called an afterimage.
Afterimages are produced when you look at an object without moving your eyes for about 20 seconds or more. During this time, the receptor cells in your eyes responsible for converting light into an electrical signal (which goes to your brain) gradually become desensitized, because they run out of the photopigments needed to conduct that signal. Your brain compensates by adapting to their decreasing signal level and interpreting it as normal.
Once the image is taken away, the least-depleted receptor cells — which had been looking at the darkest part of the image, taking in the least amount of light — jump into action at full strength. Your visual system is overwhelmed by their input, and briefly interprets it as a negative of the original image, even when it's not there.
This works to produce negative afterimages of black-and-white objects, but it can also produce inverted color afterimages because of the way our eyes perceive color. We have three different color-sensitive receptors (called cones), which perceive blue, green, and red. They work in combination to register other colors, just as we can mix these primary colors on a palette to produce any color.
If your blue receptors get tired out by staring at a blue image, the afterimage will be the inverted color: orange. If you exhaust your red receptors, the afterimage will be green.
Try it yourself with this inverted American flag. Stare at a star without moving your eyes for about 20 seconds, then look away. You'll briefly see an afterimage of the normal flag.
In the video, Wiseman created the same illusion by inverting all the colors for the first minute or so and asking you to stare at the dot. This tired out your eyes, and when he flipped the image to black and white, you briefly saw an afterimage of the original, full-color scene.