Adaptation can be specific for chromatic polarity. For example, adapting to isoluminant red spots on a gray background reduces the vividness of subsequently viewed red spots more than that of green spots (Beer and MacLeod, ARVO 2000). Becker and Anstis (OSA-UCI 2001) have demonstrated a dramatic, complete polarity specificity for luminance metacontrast masking of a spot by a ring: white rings mask white spots and black rings mask black spots, but opposite-polarity spots are not masked. This suggests fully independent ON and OFF channels for luminance perception. We now show that masking of isoluminant colors is also polarity-specific: while isoluminant red (or green) rings do indeed mask red (or green) spots, there is very little cross-polarity masking. Red rings have little effect on green spots, and green rings have little effect on red disks. This selectivity is nearly as strong as that for Becker & Anstis's luminance stimuli, and much stronger than in Beer & MacLeod's pattern adaptation. These results provide additional strong evidence for polarity-specific chromatic signals for suprathreshold colors.