Here’s a trippy one. See a color you’ve never seen on your computer screen before. True cyan.
The drill: stare at the white dot in the red circle for a couple minutes. Focus, keep staring, and keep your head still. You’ll see the inner edge of the red circle begin glowing to a hot red, with a cyan glow around the outer rim. Keep your eyes locked on the white dot and slowly back your head from your screen. You’ll see the cyan glow very hot.
The blue/cyan color chart to the right is not part of the illusion; it’s there for you to compare to the cyan glow you’ve just seen, and to prove that what you’ve seen is not part of your monitor’s color palette.
The cyan glow you’ve witnessed, or think you’ve witnessed, is the complementary color of the red. You can enjoy more geeky information on color theory here.