Results from the ground-based photometry of NGC 6791. Panel a: Δ_{C U, B, I} vs. Δ_B, I ChMs of NGC 6791 (red points) and the metal-rich GC NGC 5927 (gray triangles, Jang et al. 2022). Panel b: reproduction of the ground-based ChM of NGC 6791; the two subsamples of 1G_A and 1G_B stars are shown as orange and cyan points, respectively. Panels c and d: I vs. B–I and I vs. U–V CMDs of RGB stars in NGC 6791, where the fiducial lines for 1G_A and 1G_B are plotted in orange and cyan, respectively. Panel e: Δ(X − I) (where X = U, B, and V) calculated at I^ref = 15.0, against the central wavelength of the X filter. The colors inferred from the best-fitting isochrones are overplotted with blue star symbols (see text for details).