Left panel: (u − g) − (g − r) color–color diagram of M13. Right panel: color − color diagram of the field stars with  [Fe/H]  = −1.6, according to M13 metallicity value. Blue line illustrates the fiducial sequence of the cluster of An et al. (2008). Red color corresponds to the stars at the base of the red giant branch. Red giant branch stars in the field are expected from our color constraint.