Macula contains a translucent screen which is illuminated alternately by a blue and purple light. The yellowish pigment covering the macula (the area protecting the fovea, which gives you detailed sight) absorbs the purple light, casting a shadow which looks like a ring with a dot in the center. This shadow appears no matter where visitors look on the screen, because it is actually in their eye. The shadow is normally not seen because the eye-brain erases it from view. The alternation in color is necessary so that the image does not disappear with fatigue.