Substrates found in standard digital color printing applications frequently contain optical brightening agents. These agents fluoresce under UV light, thus increasing substrate reflectance in the short wavelength regime. The fluorescence phenomenon poses a considerable challenge in standard color management applications. This research presents a method of beneficially exploiting this phenomenon for a different application, namely watermarking. Information can be embedded in a printed color image that is perceptually invisible under normal illumination, and revealed via substrate fluorescence under UV illumination. The watermarking problem is formulated as an optimization problem that seeks pairs of colors exhibiting a close match under normal light, while producing visible luminance contrast under UV light. Models for predicting color under normal and UV light are described, and several successful watermarking examples are shown. From a practical standpoint, the approach requires no special colorants or media, and therefore can be offered at no extra cost to the user. Decoding of the watermark is easily accomplished with a common portable UV lamp.