Top: Examples of unbiased stimulus palettes with a fixed luminance contrast and no chromatic contrast (monochrome), or a fixed range of chromatic contrast along the LvsM, SvsLM, −45, or +45 deg axis. In the experiment each array was chromatically biased by adding a random uniform mean chromaticity to the palette, and observers adjusted this mean along the stimulus axis and the orthogonal axis until the palette appeared unbiased. Bottom: example in which the stimulus had a fixed range of chromatic variation along the horizontal LvsM axis (white bar) but with an added random mean shift along both the LvsM and SvsLM axis (reddish purple circle). The mean was adjusted along both the LvsM nd SvsLM axes so that the mean appeared achromatic (gray circle).