β vs. T_eff (upper left panel), [Fe/H] (upper right panel), and V magnitude (lower left panel). The results of the global fit vs. the observed β color is presented in the lower right panel. Symbols are as in Fig. 1, except that two curves are presented for MARCS models: the shallower curve is the one computed by us while the other one is from O09. Neither curve is able to explain the low β colors observed for [Fe/H]  <   − 0.2.