The pink dots appear to dissappear for exactly the same reason that the green dots appear to appear.

This is a very old trick, but I've never seen this presentation. I have seen static images with instructions to stare for thirty seconds, then stare at a neutral background to see the negative image.

When light hits the receptors in your retina, a chemical reaction takes place, which depleats an enzyme while sending a message to the optic nerve.

This enzyme slowly rebuilds itself, and our brains are wired so that we don't see this happening. In this example, the enzymes in the red and blue receptors are depleated. You see background because your brain fills it in, when you're actually seeing pink. When the pink disappears, the green receptors get much more activity, with their non-depleated enzymes, than the red or blue.

This enzyme buildup and depletion with light reception also causes your retina to become much more sensitive to light after exposure is minimized, well after the iris is completely opened.

I'm not a doctor. The above is probably all wrong. I learned this in high-school biology classes, a long time ago in a galaxy far, far away.

[edit to correct spelling]