RE: another excellent illusion...
actually its easily explained...
the ring consists of pink dots, and at every moment, 1 pink dot would be missing from the ring. the "missing" pink dot moves around in a circle
your eyes have colour detecting cells at the back of your eye, forming part of the retina. they detect the colours of everything you see
as you look at the screen at the +, or anywhere else, as long as you keep your eyes steady, the light from the circles fall on the same part of your eye, namely, the fovea where the picture is focused. hence, the cells detecting the colour pink quickly get tired, and hence when the "missing" pink dot falls on your eye, the cells detecting pink does not get activated as strongly as all the other cells, hence, white-pink= green, and you see green.
however, following the "missing" pink dot causes your eye to move, allowing light to fall on different parts of the eye. hence, the cells do not tire as quickly.
you can show that your eye cells recover pretty fast. look at the picture and blink rapidly, allowing your eyes to recover with each short blink. you will see no green dot.