Move the cursor over the image below to place a mask over the cubes, showing only the yellow and blue squares...
(Note that the mask might take a little while to load: the images are fairly large.)
An explanation of the effect can be found on the adaptation page. The squares really are grey, however: the mask isn't doing anything at all to the blue and yellow colours. As an exercise, now that you know this, try to see the grey colour when the mask is removed.
