Two powerful grouping principles in visual perception have been studied rather frequently, namely, regularity and proximity. In many cases, however, the impact of either regularity or proximity was examined. The goal of the present research is to provide and test an experimental paradigm on the basis of which the relative strengths of these grouping factors can be studied. The proposed paradigm is based on early Gestalt observations that colors of local elements within a perceptual whole tend to look more like each other than they actually are. Three experiments were set up to test whether this phenomenon of color assimilation can provide a means to compare perceptual groupings induced by regularity and proximity. In all experiments, the stimuli were linear configurations of colored elements with variable distances between the elements. The task of the participants was to compare the color of a single test element with the colors of two different groups of elements. In Experiment I, for all colors, the color of the test element was judged to be more similar to the color of the elements with which it can be grouped by means of regularity. In Experiment 2, for specific colors, the color of the test element tended to be more similar to the elements with which it can be grouped by means of proximity. Finally, in Experiment 3, configurations with a greater variety of positions were tested, including those yielding conflicting groupings, that is, on the basis of regularity and proximity different groupings were to be expected. The color judgements for these stimuli clearly reflected the competition between the two factors. (C) 1997 Elsevier Science B.V.