Here is what is happening: You have basically have 4 types of light receptors in your eyes. One is rods. They are very good at detecting motion, not color or detail. Then there are 3 types of cones: red, blue, and green receptors. These are the primary additive colors (the red, yellow, and blue you know in art to be primary are subtractive, the difference is unimportant right now, all you need to know is that you can combine red, blue and green light in any combination to make any color). The first picture is a negative of the real picture. A negative of a color is the exact opposite color. For example, the opposite of red would be a combination of blue and green. So, what happens when you stare is the color receptors of the negative color become fatigued and fire less often. Then afterwards, because those fatigued color receptors fire so in frequently, the brain perceives the opposite color. Illusions like this are called afterimages. This all has to do with the Opponent Process Theory. I would suggest looking it up, I think it is pretty cool.