The wavelengths of several constant hues over four illuminants (D95, D65, D50, A) are derived from several sets of published data. In the plane of wavelength and reciprocal illuminant color temperature (MK-I), the wavelengths of constant hues plot straight approximately parallel lines whose mean slope is about 87 degrees. Parallel lines give invariant wavelength ratios, hence constant hues in this plane are near-invariant wavelength ratios across illuminants. As recently demonstrated, the complementary wavelengths to a constant hue (across illuminants) represent the complementary constant hue; these complementary wavelengths also plot a near-parallel line to the first constant hue. To confirm and further define the constant slope of these lines, it is shown that complementary wavelength pairs, per CIE data, can only plot parallel straight lines at the angle of 87 degrees +/- 1. In summary, near-parallel sloping lines represent constant hues at near-invariant wavelength ratios. This mechanism of color constancy is shown to relate to the well-known theory of relational color constancy from invariant cone-excitation ratios. In the visual process, the latter ratios are presumably the source of the former (invariant wavelength ratios). (C) 2008 Wiley Periodicals, Inc.