Stare steadily at the dot in the heart for 30 seconds, then look at a piece of white paper. A faint Red heart surrounded by a bluish outline should mysteriously appear on the paper.
This is called an aftereffect. Staring at the heart overstimulates your eyes' photoreceptors, which are what absorb the light that stimulates vision. So even after you look away from the image, your brain is still receiving messages about it. What you end up seeing are called complementary colors. Red and blue are complementary colors of green and yellow.
- Al Seckel