Sensitivity to the number of colors in textures defined by luminance and chromatic contrast

Changes in the mean or variance of a color ensemble can be readily detected, but how sensitive are observers to the actual number of colors defining the array? Discriminating changes in the number of unique hues or saturations in the set becomes difficult when the set reaches four or more levels [J. Opt. Soc. Am. A42, B413 (2025)JOAOD60740-323210.1364/JOSAA.31.545299]. Here we extended these results to examine palettes that vary in both luminance and chromaticity. Stimuli were textures of 11×11 square elements, with the color of each element chosen randomly from a palette. The palettes consisted of colors that were evenly spaced along different planes in cardinal axis color space (LM, S, and achromatic). In the LM versus S plane, elements varied only in hue angle at constant chromatic contrast, while in achromatic versus chromatic planes, the sets varied in both luminance and chromatic contrast. Four textures were displayed on a monitor and observers judged which one contained an additional color. Accuracy degraded as the set size increased beyond three colors. However, when the textures included different luminance levels, the set size differences remained more visible. This advantage persisted over large changes in the maximum luminance contrast and was accentuated when pure black or white elements were present, potentially reflecting sensitivity to pure achromatic features. The results suggest that, while numerosity judgments of color are in general very restricted, different attributes of color (hue, saturation, and brightness) exhibit different properties in texture perception, with enhanced sensitivity to luminance features.