We present a watermark encoding and detection method by which colorant combinations and narrowband light sources are selected to optimally differentiate between colorant combinations that produce the same visual color under wideband illumination. Thus colors that look the same under white light can reveal additional information (e.g. watermarks) under special narrowband illumination. These different colorant combinations produce metameric matches; that is, colors that are spectrally different but that integrate to the same color for a human observer under a particular light source. The metameric colorant combinations and narrowband illumination can be jointly selected to allow sufficient or maximal discrimination under the potential narrowband illuminants. Alternately, for a given colorant combination that achieves sufficiently similar appearance under wideband illumination, narrowband illumination such as LEDs (singly or in combination) can be optimally selected to reveal colorant differences.