This disclosure describes methods for using embedded auxiliary signals in documents for copy detection. In one implementation we embed an auxiliary signal with a first color that is out of gamut with process color inks or with a particular range of printers. A process color approximation of the first color yields a relatively different contrast when, e.g., viewed with an interference filter. A copy is determined based on the relative presence or absence of the auxiliary signal. In another implementation, we provide a first auxiliary signal with a first color and a second auxiliary signal with a second color. A filter is matched according to the spectral response of the first color. The second color preferably includes a spectral response that is inversely related to the matched filter. The first signal is then detectable in a process color copy, but less so in the original. However, the second signal is detectable in the original, but less so in the copy. In still other implementations we compare relative contrast of a signal to determine an original from a copy.