Color–color diagrams of the 65 group 1 clusters (filled dots) and 138 group 2 clusters (open dots) defined in Fig. 3. Panel a) shows the situation in optical colors only, while panel b) shows the situation when we make use of the ultraviolet and near-infrared filters. In both panels, the error bars indicate the maximum photometric error for the group 1 clusters. The three lines show SSP evolutionary tracks of metallicity [M/H] = 0 (dotted line), −1 (dashed line), and −2 (solid line). The SSP ages extend from log ₁₀(t/ yr) = 6.6, in the upper part of each panel, to 10.1 in the lower part of each panel. The arrows indicate the direction of the extinction A_V = 1.