Hue can be described by four separate sensations of red (R), green (G), yellow (Y), and blue (B). These are combined in the spectrally opponent RG and YB mechanisms, whose null points correspond to the unique sensations of Y, G, B. Participants used a form of magnitude estimation to describe color appearance of light flashes that were systematically varied in size, luminance, duration, purity, and retinal eccentricity. Wavelengths of the unique hues were derived from the hue and saturation scaling functions. Only unique Y remained invariant across all the viewing conditions. The shifts in unique hues with test conditions place strong restrictions on models of how the RG and YB mechanisms are assembled. Despite polymorphism of L and M cones and variation of their ratios across participants and across the retina, the frequency distribution of unique Y was very narrow, implying some reweighting of cone inputs to individuals' RG mechanisms.