1. A method for encoding an information signal in spectral differences within an image signal, the method comprising, with a programmed processor: generating an information signal comprising message symbols; mapping message signals of the information signal to insertion locations within a host image signal; at the locations, inserting the information signal by computing values for color components at the locations so as to form a spectral difference signal between ink applied at a first pixel location and ink applied at a second pixel location to encode a message symbol in relative spectral difference between the ink at the first and second pixel locations, the computing encoding relative spectral differences at the locations that conveys the information signal within the host image, the spectral difference signal representing a distinguishable spectra between two or more colors that appear similar in the human visual system at the insertion locations within the host image signal, wherein the relative spectral differences comprise differences between spectral distributions of inks at corresponding pairs or sets of locations.