Behaviour of the average difference between the spectroscopic and photometric T_eff obtained from individual colours for target clusters (red circles) as a function of the iron abundance derived adopting spectroscopic parameters. The values of T_eff in the upper and lower panels were obtained with the relations of González Hernández & Bonifacio (2009) and Alonso et al. (1999), respectively. The vertical errorbars are the dispersions of the mean for T_eff for each cluster. Thick grey lines are the best linear fits obtained for the clusters with [Fe/H] < –1.5 dex.