Globular clusters in M 31. The (u − g) versus (g − r) colors of globular clusters in M 31 are shown, color coded by their iron metallicity, as reported in Caldwell et al. (2011). To minimize the effect of dust, the sample has been reduced to those globular clusters with E(B − V) < 0.15. The gray background grid shows the FASTAR photometric predictions for different ages and metallicities, for a fixed Milky Way-like IMF.