Human eyes are often “cheated” by an optical illusion (or visual illusion) so that the perceived image differs from the physical reality. But various optical illusions have been seldom investigated for technological applications such as image processing and optical display in the past. As a unique attempt of combining information technology with optical illusion, we propose a novel image steganography scheme based on a color assimilation illusion. A synthesized image containing a grayscale background and a saturated color line (or point) grid can be perceived as a color image, with external secret data hidden simultaneously.