Read this first: If you follow the instructions below, you will change your brain for a prolonged time (up to month), in addition to simply remembering this. Proceed only if this is ok with you.
What to see
Initially you will see a simple 2×2 checkerboard consisting of horizontally and vertically striped squares. And the stripes will be black-white, no colour. Fine so far. You may want to try out the radio button “Simulation”, which will faintly colour the white stripes.
What to do
Select the radio button “Adapt”. You will now see coloured stripes, two sets, alternating every 2 seconds. This needs to be viewed for 2–5 minutes. Yes, 2 minutes at least. To help you make it through this time, select a piece of your favourite music (of appropriate duraton) to play along, and try to figure out what the letters mean that appear every second. The line below the letters gives current letter number / total number of letters.
When you feel you have adapted sufficiently long (no need to pay attention to colours or stripes, just follow the fixation letters), switch back to the radio button “Test” (or press Reset). Now you will see faint colours in the pattern that has been black and white before. Nothing to write home about? Just wait: All other aftereffects (be they in brightness, colour or motion) last at most a minute. This one lasts hours! Or overnight. Or a week. Adapting for 10 minutes is supposed to give a lasting effect over months.
If you rotate your head or the screen, you can observe that the colours will attach to the other set of lines.
Aftereffects are commonly observed in your daily life, e.g. after looking into the sun (very briefly!). The McCollough Effect, as demonstrated here, lasts so long that it must belong to an entirely different category. To me it belongs to “learning”, like learning a poem or a musical instrument. Further, it it is a contingent phenomenon: the colour binds to the orientation of the grating: Green was horizontal, and the horizontal stripes show the faint magenta aftereffect.
The mechanisms of the McCollough Effect are not understood in spite of 50 years of research. Her original suggestion, namely that it is related to the removal of colour fringes, which occur in our eyes all the time yet we don't see them, appears very plausible to me, but some experiments are not fully compatible with it (reviewed in the excellent Scholorpedia article by Michael Webster and Celeste McCollough).
McCollough C (1965) Color adaptation of edge-detectors in the human visual system. Science 149:1115–1116 [PDF]
McCollough Howard C, Webster WA (2011) Scholarpedia, 6(2):8175