That's really cool, I had tested it before but only with white. I wonder if it could also complete a numeric sequence, that would be quite telling on what part(s) of the brain is involved in completing the picture. Is it just image processing, or are higher cognitive functions involved? I'll try it soon, although it's hard to prove anything when you design the experiment yourself.

However, it doesn't mean everything we see is an optical illusion. We have two eyes for a reason, plus in a normal situation our eyes are always moving around in order to get a good picture of everything around you.

The brain is awesome. What it actually does is creating a 3D world around you by two incomplete 2D images from your eyes. And even those 2D images are not similar to normal images but made up of edges. Give us a white plane and the eye sees almost nothing, it mostly sees edges. But our brain computes an image. And besides a small area in the center of our vision we do not see colors either.