Color–color diagrams for RGB stars at different metallicities generated with the CMD 3.9 web interface. Gray contours show the observed RGB sample adopted in this work with foreground MW extinction corrected (darker shades indicate higher tracer density), and the red arrows indicate the reddening vector for A_V = 1 mag assuming the mean extinction law of M33 (Y. Wang et al. 2022). The sharp cutoff at F475W − F814W = 1.74 reflects the exclusion of the bluest 5% of tracers in F475W − F814W to reduce contamination from helium-burning and asymptotic-giant-branch stars, as described in Section 2.