meelevenseconds.com

you can read about the theory behind this effect here and here

PART ONE: hide in black and white

type in a short phrase and press the button. an image will be generated displaying the phrase. the phrase will disappear if you render the image in black and white (for example, if you print it out on a black and white printer)

PART TWO: hide in color

type in a short phrase and press the button. an image will be generated displaying the phrase. the phrase will appear prominently if you render the image in black and white and disappear if you render it in color (while your screen is a color screen, you may still see the image because of your browser's image color correction or color variability of your LCD screen)

and a distraction text: