1. An image processing apparatus, comprising: a ground pattern detector for detecting whether a ground pattern is included in an image input; a latent image extractor for extracting a latent image included in said ground pattern from a ground of said image input when said ground pattern detector detects the ground pattern form said image input; a lightness corrector for correcting lightness of said latent image to become closer to lightness of an element of said image other than said ground; a latent image converter for converting a color of said latent image having corrected lightness into a first plurality of colors formed with a plurality of different colors excluding a background color for outputting and varying from one to another via a neutral color; and a converter for converting a color of said element of said image other than said ground into a second plurality of colors formed with said plurality of different colors as said first plurality of colors and varying in a manner different from that of said first plurality of colors.