As Fig. 2, but for synthetic samples of P1 stars (200 objects) including a 0.15 dex uniform spread of metallicity (left panels), or a uniform spread ΔY = 0.05 (right panels). The top row displays the results for populations with [Fe/H] distributed between −2.15 and −2.0 dex, and for a spread of helium at [Fe/H] = −2.15. The middle row shows the case for [Fe/H] between −1.6 and −1.45 dex, and for a spread of helium at [Fe/H] = −1.3. The bottom row displays the case for [Fe/H] between −0.85 and −1.0 dex, and for a range of helium at [Fe/H] = −0.7. In each panel the solid vertical lines at Δ(F275W − F814W) = 0 are the verticalised red fiducials of the F814W-(F275W − F814W) CMDs of the samples. The other line in each panel corresponds to the blue fiducial of the Δ(F275W − F814W) distribution as a function of F814W (see text for details).