1. A method for performing a fully computerized and accurate color separation of an input color image into a color separated output image printable on a substrate with at least one metallic ink comprising the step of fitting surface coverages of the metallic ink and of transparent inks according to a model predicting the color of superpositions of the metallic ink and the transparent inks, said model relying on the transparent inks' internal transmittances and on the substrate's reflectance, where said color separated output image is printed on an item selected from the set of security documents and valuable products and where said color separated output image comprises superpositions of the metallic ink and the transparent inks at specularly reflecting locations and superpositions of transparent inks without metallic ink at other locations, and where the specularly reflecting locations of the superpositions of the metallic ink and the transparent inks are hidden at non-specular reflection angles and become visible at specular reflection angles.