This one is a computer trick, about 30 seconds after going to the page,
whether you stare at the dot or not, moving the mouse onto the picture
changes the picture.

Staring at the pink dots selectively bleaches the retina of the
pigments responsible for detecting pink without affecting those that
detect green.  You can test this by looking away from the cross, you
will see a pattern of green dots.

The computer program sequentially flashes the pink dots off and on in
order, the green dot is seen when the pink dot is missing.

good ones!!!

my guess is:
if an image is there on retina, it gets saturated and disappears.
the second one is if the saturated retina is turned elsewhere
(or the color changed) a sort of negative color takes its place
(color perceived by the unsaturated cones), that sort of 'flips' the
color
of original.

so:

in the first castle pic, the original pic is already in a sort of
negative/flipped/
one color missing mode. fixating on the dot will saturate retina and
then
when moused over, the image is changed to B&W *but* for the retina the
color will be flipped. hence the blue sky and green grass.
[if one stares at the dot long enuf after mousing over, the image comes
to B&W]

second is similar wrt pink dot becoming green, additionally, fixating
on the pink
dots long enuf saturates the retina and they disappear.