The responses of opponent channels have been modeled in the past as a linear transformation of cone absorption values L, M, S. The authors asked two related questions: (i) which form of transformation is psychologically most plausible and (ii) is a linear transformation the right model, in the first place. The authors tested positions of unique hues for seven subjects in an xy chromaticity diagram as well as in a Boynton–MacLeod chromaticity diagram in log-coordinates. The results show that neither of the two opponent channels can be adequately approximated by a single straight line. The red-green channel can be approximated by two straight lines. The blue-yellow channel can be approximated by a quadratic function, whose middle section coincides closely with the daylight locus. These results show that linear models do not provide an adequate description of opponent channels. Our further analysis shows that there is a correlation between the red and the green directions.