Photometric metallicity distributions (upper panels) and comparison with spectroscopic measurements (lower panels). From left to right, the photometric metallicities were obtained using Eqs. (2), (5), (3), and (4). Light histograms in the upper panels show the entire clean RGB star sample, dark histograms show only stars with y₀< 19 mag. Each panel in the upper row reports, for the brighter subsample (y₀< 19 mag), the sample mean μ_sample and standard deviation σ_sample, as well as the intrinsic mean μ_intr and dispersion σ_intr obtained via Eq. (8).