For normal trichromats, the hue of a light can change as its
luminance varies. This Bezold-Brücke (B-B) hue shift is commonly
attributed to nonlinearity in the blue–yellow opponent system. In
the present study, we questioned whether protanopes experience
analogous changes. Two protanopes (Ps) viewed spectral lights at six
luminance levels across three log steps. Two normal trichromats (NTs)
were tested for comparison. A variant of the color-naming method was
used, with an additional “white” term. To overcome the
difficulty of Ps' idiosyncratic color naming, we converted
color-naming functions into individual color spaces, by way of
interstimulus similarities and multidimensional scaling (MDS). The
color spaces describe each stimulus in terms of spatial coordinates, so
that hue shifts are measured geometrically, as displacements along
specific dimensions. For the NTs, a B-B shift derived through MDS
agreed well with values obtained directly by matching color-naming
functions. A change in color appearance was also observed for the Ps,
distinct from that in perceived brightness. This change was about twice
as large as the B-B shift for NTs and combined what the latter would
distinguish as hue and saturation shifts. The protanopic analogue of
the B-B shift indicates that the blue–yellow nonlinearity
persists in the absence of a red–green signal. In addition, at
mesopic levels (≤ 38 td), the Ps' MDS solution was two
dimensional at longer wavelengths, suggesting rod input. Conversely, at
higher luminance levels (76 td–760 td) the MDS solution was
essentially one dimensional, placing a lower limit on S-cone input at
longer wavelengths.