Consider it this way, Ian. If you took the individual dots, apart from the rest of the illusion, and looked only at the dots or photographed them, or ran them through a photospectrometer, they would be pink. The green that you imagine is situational, depending on the presence of other things and on the fact that you must not look directly at them (or they become pink again).
However, they are both part of reality, its just that the "green" part of reality is only in your brain, not in the objects themselves. The fact that your brain states can be so easily manipulated does not negate materialism, on the contrary it shows just how the material aspects of your brain and sensory organs work.
In one sense, they are like dreams. The things you dream are often not real, but your brain states are real and your memories of your dreams are real. Even without looking at that illusion again, I can remember the way they pink changed to green and orbited the center. It doesn't matter that my memory is of something that didn't really happen.