I am going to make a guess, tell me if I am close? Okay, the green is the negative of pink, and when the cones in our eyes get zapped, they are momentarily 'depleted' and the opposite, or negative, color appears as an after effect. When we stare at the middle, everything is staying in the same position, and the pink dots also induce the negative - green - which we see if we move our focus. When the pink, and the green 'after effect' happen almost simultaneously, the two colors added together equal that grey color that is the background. I think it must be made to happen correctly by the timing of the flashes as the dot goes round. Perhaps?
I know that we see in frames like a movie, not continuous motion. Ever look at a spinning hubcap and it starts to look like it's spinning backwards?
doh, didn't see that. crazy rob - guess I should have said "the only optical illusion that actually gave me an acid flashback."