Abstract. In this paper we present a new efficient watermarking scheme for images. The basic idea of our method is to alter the colors of the given image in a suitable but imperceptible way. This is accomplished by moving the coordinates of each color in the color opponency space. The scheme is shown to be robust against a large class of image manipulations. The robustness of the scheme is also theoretically analyzed and it is shown to depend on the number of colors in the image being marked.