1. In a printer in which at least two printing stations cause images to be printed on a substrate, said printer being capable of registration adjustment, method for monitoring registration of images printed thereby, the method comprising: (a) printing a pattern of spaced black registration marks onto said substrate by operation of one of said printing stations; (b) printing a pattern of spaced non-black registration marks onto said substrate by operation of a second of said printing stations, said pattern of spaced non-black registration marks being superimposed on said pattern of spaced black registration marks to form a composite pattern of registration marks and interposed spaces; (c) illuminating said composite pattern with at least one LED having an output wavelength complementary to said non-black color; (d) examining the reflectivity of said composite pattern at wavelengths complementary to said non-black color to obtain a reflectivity signature for said composite pattern; and (e) comparing said reflectivity signature to a pre-determined reflectivity signature for said composite pattern, indicative of good registration, to determine an adjustment factor for said printer.