Primary visual cortex contains at least two distinct populations of color-selective cells: neurons in one have circularly symmetric receptive fields and respond best to reddish and greenish light, while neurons in another have oriented receptive fields and a variety of color preferences. The relative prevalence and perceptual roles of the two kinds of neurons remain controversial, however. We used fMRI and a selective adaptation technique to measure responses attributable to these two populations. The technique revealed evidence of adaptation in both populations and indicated that they each produced strong signals in V1 and other human visual areas. The activity of both sets of neurons was also reflected in color appearance measurements made with the same stimuli. Thus, both oriented and unoriented color-selective cells in V1 are important components of the neural pathways that underlie perception of color.