Since the colorant halftone separations are printed sequentially and overlaid on the paper substrate, per-channel embedding and detection of watermark patterns is an attractive option that would extend monochrome watermarking methods to color. To detect the watermark patterns, one would like to acquire the constituent halftone separations used in the color printing system from the (overlaid) print. This would work, for example, if one could deploy a scanner with N color channels, where each color channel captures only one of the N colorants used in the printing system. In actual practice, desktop scanners commonly use RGB color filters to capture color. For three-color CMY printing, as illustrated in Fig. 1, there is a complementary relationship between the CMY colorants and RGB scanner channels: the cyan (C), magenta (M), and yellow (Y) colorants absorb light, respectively, over the spectral regions in which the red (R), green (G), and blue (B) scanner channels are sensitive. Thus, in an ideal setting, C, M, and Y colorant halftones may be estimated from the scanner R, G, and B channels, respectively. In practice, however, this is usually not feasible for a couple of reasons. First, typical printing systems utilize CMYK (four) colorants. The black (K) colorant absorbs uniformly across the spectrum, and thus it consistently appears in the scanner RGB channels. Second, so-called “unwanted absorptions” of the CMY colorants also cause cross-coupling, i.e., C, M, and Y halftone separations not only appear in the scan R, G, and B channels that complement their spectral absorption bands, respectively, but also in the two other channels as well. The image of Fig. 2 illustrates the couplings between the CMYK halftone separations in the scanner RGB channels. Due to these undesired couplings, the four C, M, Y, and K halftone separations cannot be directly obtained from individual R, G, and B scanner channel responses.