Comparison between the B vs. U − B (panels a)), the V vs. V − I (panels b)) and V vs. c_U,B,I (panels c)) CMDs of NGC 6121 (up), NGC 6397 (middle) and NGC 6752 (down). Red triangles and blue points represent the groups of rMS and bMS stars defined in panels a). The insets of panels a) show the Δ(U − B) distributions, fitted with a bigaussian in the case of NGC 6121 and NGC 6752 and with a Gaussian in the case of NGC 6397. The horizontal bars show the mean error in color.