What Color is the Stripe?
Stare at the black dot in the bottom right-hand corner and press "play". It will appear that the moving gray bar begins to change color. For some colors and tones, you need to focus on the black dot for a couple of seconds before the illusion becomes apparent. This illusion, like the lilac chaser illusion , relies on peripheral vision to trick your brain into perceiving the color complementary to the inner background.
 Wikipedia. "Lilac Chaser." (Feb 8, 2016) en.wikipedia.org/wiki/Lilac_chaser.
"What Color is the Stripe?"
Wolfram Demonstrations Project
Published: February 15 2016