The numerical index system of color harmony is intended to mark a great number of color pairs, optimally any number of existing color pairs, by a number between 0 and 100. This number expresses the extent to which a color pair is being felt harmonious by the average of people and the level of harmony content it possesses. The experiments described in this article have determined the basic data necessary to create this system. The series of the experiments have been done in two stages. The first stage, in which 24 test objects were presented to the experimental subjects, was carried out twice first in 19881990 and again in 20042006. Every test set was composed of eight compositions. The number of scores, given to each of the compositions, determined the harmony content of the color pair groups, whose members are formed from the saturated colors of different hues and from the members of the grey scale. In the second stage of the experiment, these data served as references for the experimental subjects. In the second stage, there were 192 tests. In these tests, there were different numbers of compositions each formed of different color pairs. One of the members of these color pairs was the member of the saturated color of the first experiment. The second member was always of different saturation and lightness for each of the compositions, purposefully chosen to match the saturated colors. Based on the experimental scores, we obtained a color harmony surface linked to the intersections with the coordinates in the Coloroid system. The color harmony surfaces and the distances between the related intersections indicate the harmony content of the color pair. The numerical values of these distances are called the color harmony index number of the color pair. These data make the creation of a color harmony indexing system possible, expressing the color harmony content of all possible color pairs, in the color space. 2011 Wiley Periodicals, Inc. Col Res Appl, 2012