A developer unit used in xerographic printing retains a mixture of at least a first colorant and a second colorant. An illuminator directs toward at least a portion of the mixture a first light of one color, a second light of a second color, and a third light substantially outside of a visible range. A photosensor records a first reflectance signal based on light reflected from the mixture substantially in a visible range, and a second reflectance signal based on light reflected from the mixture substantially outside a visible range. The signals can be used to maintain the mixture at the desired total color and other properties.