Copying of visual images by electronic devices is discouraged by metamerically altering the relative weights of the color components forming the visual images in a manner that is not perceptible to the human eye but which is perceptible in the copy. In a preferred technique, the visual images are represented by more than three primary color components whose relative values are varied in this manner. A primary application is to discourage copying by video cameras of movies displayed on theater movie screens, by causing the copies to contain unacceptable spatial or temporal variations in colors perceived by the human eye.