Spanish Castle Optical Illusion

OK, it's an optical illusion.

There's this picture of a castle which looks kind of yellow and blue. You stare at it for 30 seconds. (You can blink.) Then, without taking your eyes off the picture, you move your mouse onto it. For an instant, it's color, then it goes to black and white.

If you move your mouse onto it without staring, it's just a black and white picture.