According to Hering's color theory, certain hues (red vs green and blue vs yellow) are mutually exclusive as components of a single color; consequently a color cannot be perceived as reddish-green or bluish-yellow. The goal of our study is to test this key postulate of the opponent color theory. Using the method of adjustment, our observers determine the boundaries of chromatic zones in a red green continuum. We demonstrate on two distinct stimulus sets, one formed using a chromatic grid and neon spreading and the other based on solid colored regions, that the chromatic contrast of a purple surround over a red figure results in perception of 'forbidden' reddish-green colors. The observed phenomenon can be understood as resulting from the construction of a virtual filter, a process that bypasses photoreceptor summation and permits forbidden color combinations. Showing that opponent hue combinations, previously reported only under artificial image stabilization, can be present in normal viewing conditions offers new approaches for the experimental study of the dimensionality and structure of perceptual color space.
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