Crazy Optical Illusion!

This is a crazy colour optical illusion. You must click on this link to access the true optical illusion. Make sure your mouse cursor is not on the image of the Big Spanish Castle. Now stare at the black dot for 15 or more seconds. Then without moving your eyes, put the mouse cursor on the image. Tada! Move your eyes again and you lose it in colour! This is what the castle looks like in black and white: