Data from Experiment 2 illustrate that forbidden hues are not peculiar to neon color stimuli. The virtual spectral transmittance (Fig. 6) is consistent with the results obtained in Experiment 2 as well as Experiment 1, assuming the solid diamond is interpreted as a transparent layer in front of the solid background, so the virtual filter explanation described for neon displays is also completely consistent with the scission hypothesis applied to solid stimuli. Unlike the neon stimuli, however, chromatic induction in the solid stimuli also readily admits of a second interpretation, with a uniform filter or illuminant that is discounted (color constancy). As discussed next, mechanisms of color constancy would produce the same results in our experiment as the scission mechanism.
The visual system could either interpret the solid stimuli in the Experiment 2 as diamond-shaped filters seen in front of a purple plane, similar to the neon case, or as diamond shapes viewed through a rectangular purple filter or purple illuminant that covers the whole stimulus. Let us designate these interpretations as the scission hypothesis and the color constancy hypothesis (because this interpretation involves discounting the filter or illuminant), respectively. Since our subjects were asked to evaluate the color of the diamond, their answers should refer either to the color of the diamond object (according to the color constancy hypothesis) or to the color of the diamond filter (according to the scission hypothesis). If the stimulus sequence (Fig. 3(b)) causes any sense of transparency, the green end of the continuum should either be interpreted as a green shape seen through a rectangular purple filter or a diamond-shaped green filter in front of a purple plane. However, none of our subjects reported any redness at this end of the continuum, whether in their chromatic boundary settings or their verbal report — the only color name used was ‘green’ here. Therefore, we can conclude that, to the extent transparency was involved at all, our subjects could effectively discount the purple color of the rectangular filter (if they perceive this stimulus as a green diamond viewed through a purple filter) or the purple color of the plane behind the filter (if they perceive this stimulus as viewed through a diamond-shaped green filter). This conclusion is consistent with perceptual experience one would get viewing a physical green diamond shape through a physical purple filter covering the whole stimulus, or viewing a purple surface through a green diamond-shaped filter. In fact, this observation is a reflection of color constancy.