Before revision:  To define the metric, we determined the minimal chromatic difference between a stimulus and its surroundrequired by each subject to detect the stimulus. Next, observers performed discrimination experiments between two spatially localized stimuli presented on a surround of a different chromaticity . The surround color affected the discrimination thresholds. Quite remarkably, when these thresholds were expressed in the color coordinatesdefined before , the change in thresholds followed a simple law that only depended on the  distance between the surround and  the two compared stimuli.
After revision:  To define the metric, we determined the minimal chromatic difference required for each observer to differentiate a stimulus from its surround. Next, we showed that this perceptual metric was particularly adequate to describe two additional experiments, since it revealed the natural symmetry of perceptual computations. In one of the experiments, observers were required to discriminate two stimuli surrounded by a chromaticity that differed from that of the tested stimuli. In the perceptual coordinates , the change in thresholds followed a simple law that only depended on the distance between the surround and the two compared stimuli.