The way in which color signals from the three cone classes (L, M, S) are handled by the rest of the visual system to bring about color perception is incompletely known. In particular, the neural mechanism underlying two fundamental features of color vision, color contrast and color constancy, are unclear. Modeling efforts have shown that these features could be accounted for by neurons capable of making chromatic comparisons across visual space. The existence of such neurons in the primate is contested. I revisited the issue, recording the activity of single neurons in primary visual cortex of alert macaques trained to fixate a dot on a computer monitor, on which I flashed small spots of light that modulated a single cone class at a time. Cone-isolating stimuli can either increase or decrease one of the three cone types, thus there are a total of 6 stimuli; the stimuli were presented on a neutral gray adapting background. I correlated the location of the spots with the neural activity to produce receptive field maps. A fraction of neurons had both spatially and chromatically structured receptive fields. These were compared with receptive fields determined using stimuli presented on different-colored highcontrast (non-adapting) backgrounds. Receptive-fields with high-contrast stimuli had the same shape, but were slightly larger (10%) and had slightly shorter (5ms) latencies. These “double-opponent” neurons respond best to color contrast and could be the building blocks for color constancy.