a) Selected field RGs from the sample of ATT94/ATT98 plotted in the [m], y − J plane (Ns = 61, filled dots). b) Difference between photometric and spectroscopic metallicities, Δ [M/H] = [M/H] _phot − [M/H] _spec, plotted versus [M/H] _spec for the 61 field RGs (filled dots). Photometric metallicities are based on the α-enhanced [m] , y − J MIC relation. c) Normalized photometric metallicity distribution for the 61 RGs obtained with the [m], y − J MIC relation (black solid line), compared with the normalized spectroscopic distribution (black dashed).