C1 colour–colour diagram sensitive to global metallicity for a set of clusters corrected for reddening using A_X absorption values in that passband derived as indicated in Appendix H. Lines represent the simulations performed using the BTSettl library (Allard et al. 2013; with a line colour depending on the global metallicity, [M/H]). Solid squares represent the stars in GCs and empty triangles the stars in OCs, all of them with their error bars. BTSettl models with log g = 2.0 are plotted for GCs, and log g = 3.0 for OCs.