Why is afterimage green when you stare at a magenta picture?

Afterimages occur when the cones in our eyes become fatigued after being exposed to a particular color for an extended period of time. In the case of staring at a magenta picture, the cones that are sensitive to green light become fatigued, causing a green afterimage to appear when you look away. This is known as a complementary afterimage, where the afterimage color is on the opposite side of the color wheel from the original stimulus.