We present a theoretical calibration of a new metallicity diagnostic based on the Strömgren index m1 and on visual - near-infrared (NIR) colors to estimate the global metal abundance of cluster and field dwarf stars. To perform the metallicity calibration we adopt α-enhanced evolutionary models transformed into the observational plane by using atmosphere models computed adopting the same chemical mixture. We apply the new visual-NIR metallicity-index-color (MIC) relations to two different samples of field dwarfs and we find that the difference between photometric estimates and spectroscopic measurements is on average smaller than 0.1 dex, with a dispersion smaller than σ = 0.3 dex. We apply the same MIC relations to a metal-poor (M 92) and a metal-rich (47 Tuc) globular cluster. We find a peak of -2.01 ± 0.08 (σ = 0.30 dex) and -0.47 ± 0.01 (σ = 0.42 dex), respectively.