Figure 1.

Stimulus color space. Observers were instructed to select four unique hues: a red and green that were neither bluish nor yellowish, and a blue and yellow that were neither reddish nor greenish. (A) Hues were selected by navigating a gradated patch of hues with a keyboard. (B) Candidate hues fell along a circle around neutral white in the isoluminant plane of cardinal color space; numbers indicate the cone coordinates (L, M, S) at axis extrema. Unique hues for seven observers are shown: red (▽), blue (□), green (○), and yellow (⋄).