Metallicity [M/H] versus Δ_F275W,F814W pseudo-color relationship for four GCs. Top panels: chromosome maps of the clusters with P1 stars within our sample color-coded by their metallicity. Middle panels: metallicity of each star, with the relationship derived from the WLS regression (red line) and the 95% confidence interval (red shaded area) over the color range ΔC. We also indicate the number of stars (N), the Pearson correlation coefficient (R_P), and the resulting metallicity variation Δ[M/H]. Bottom panels: residuals (i.e., observed minus predicted metallicity). We indicate the standard deviation (σ) of the residuals.