A central question in color vision is how neural activity due to spectral differences in the stimulus is integrated to produce the color perceived of an object. The problem is complex because in a sufficiently complex environment, the color attributed to a surface covaries more closely with its reflectance than the spectral characteristics of the light reaching our eye from the surface, itself. However, in the case of isolated fields in a dark surround, it might be thought that a simple relation between neural coding and color could be defined. While Young's notion of fundamental hue sensations associated with the excitation of individual classes of receptors has long since been discarded, linear and some nonlinear models relating photoreceptor excitation and color predict just such an association in this simple situation. For example, in the Hurvich & Jameson (1968) opponent-colors model, L-cones signals contribute redness and yellowness, M-cones greeness and yellowness, and S-cones blueness and redness. Nevertheless, this assignment is not uncontroversial, as several studies have suggested that M-cones could signal blueness, under some conditions. Knoblauch & Shevell (2001), using a hue-cancellation task in which the modulation of lights was constrained along cone-isolating directions, found that sometimes yellow-blue cancellation yielded non-monotonic relations between cone-excitation and the interaction of opponent-colors. These results are inconsistent with fundamental hue sensations associated with cone signals and suggest a resolution to some conflicting claims about what colors cones signal. Additionally, the non-monotonic relations are consistent with the rectifying nonlinearities observed in sub-cortical chromatic coding.