Take a look at the sequence of images below (I recommend clicking through to enlarge, Tumblr Dashboard folks). Works best if you stare at the flowers:
Color? Or black and white?
The rods and cones of your retina respond to the illumination (black-white) and color of a scene,…
For the best effect, stare at the top of the centre tulip and don’t move your eyes at all (if you don’t try hard to keep them still, they will automatically dart around a little, performing what are called ‘saccades’).