UVI colour-magnitude diagram of GC candidates with photometric error ≤0.07 mag in each band (dots). The lines correspond to stellar tracks of constant age (dashed) and constant metallicity (solid) derived from the PARSEC models (Bressan et al. 2012). The black ellipse represents the mean errors in colour. We select three subpopulations of GCs based on the PARSEC tracks: young, metal-rich (light blue), old, metal-rich (orange), and old, metal-poor (magenta).