Patches are generated from the overprinting of one or more separations to encode four or more values per patch each color representing a numerical value. In a calibration step, all possible color overprint combinations are printed multiple times on a calibration sheet. The sheet is scanned and each overprinted patch is entered in to a proximity array, which is a volume data structure representing three dimensional (3D) color space). After all calibration colors are entered, the proximity array is repeatedly dilated. Later, sheets that need to be identified are printed with a code number encoded as one or more patch codes. The sheet is scanned and the patch code colors are looked up in the proximity array to determine which original colors were printed. The identified colors are converted the code number. Patch codes are further augmented with error detection and optionally error correction bits to further increase robustness.