Luminance-dependent change in color appearance—the Bezold–Brücke effect—was investigated in protanopes and related to that in normal trichromats. Spectral lights were presented at six luminance levels covering mesopic, low, and high photopic vision—across three log steps from . To judge color appearance, a variant of the color-naming method was used with four primary basic color terms and a “White” response. This modification enabled us to examine apparent saturation changes along with the Bezold–Brücke hue shift. Color-naming frequency functions were acquired across ten presentations of each stimulus. Since protanopes name colors idiosyncratically, changes in color appearance cannot be quantified directly from the color-naming functions. To circumvent the difficulty, these functions were transformed into color similarity measures for analysis with multidimensional scaling purported to reconstruct individual color spaces. In these, luminance-dependent shifts in color appearance were represented by means of geometric displacements. We found that for normal trichromats, shifts measured in this way agreed with those derived in our study directly, and with the hue shifts reported in earlier studies. For protanopes, contrary to some models of dichromatic vision, changes in color appearance are significant and indicate superimposed shifts in hue and saturation. The results obtained for normal trichromats, especially for protanopes, imply that nonlinearity in the yellow–blue opponent system is insufficient to explain the Bezold–Brücke effect, given the nature of the saturation shift and the demonstrated divergence between unique hues and invariant hues.
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