If you sit back and think about it, the human eye is a pretty crazy thing. From the front lens all the way back into your brain, the mechanisms that allow us to see the world around us are simply incredible. And with some knowledge, you can fool with that system. In this case, you can use something called opponent process to make your brain see a black-and-white image as full-color.
The process works like this: You’re shown a version of the image where each color is replaced with a complementary color. Different types of receptors in the eye are sensitive to different pieces of the visual spectrum. When they’re stimulated and then the stimuli is removed, there’s a lingering effect that we perceive as a shape of the opposite color.
It’s a fascinating phenomenon and one that has been known for quite some time. I remember my grandmother having a small slip of paper with some blobs on it. If you stared at it long enough and then looked at a blank wall, you’d see an image of Jesus.
Still, next time someone shows you the cool trick, you can bust out your new Wikipedia knowledge of opponent process and impress everyone at the dinner party.