The color aftereffects phenomenon predicts that, after staring at a bright red rectangle for a period of time, you will see a when you look away at a white background.

The answer for this is Green Rectangle.

Explanation:

The hue after effects theory suggests that as you glance away at a white background, you can see a Green Rectangle after looking at a bright red rectangle for a period of time.

All this is an after-image consequence. Afterimage is a form of optical illusion in which, even after exposure to the original image has ended, an picture appears to appear momentarily.