Difference (Δ(BC)) between BCs calculated for P1 and P2 compositions (P1-P2), at selected points along a 12 Gyr isochrone with [Fe/H] = −0.7 and Y = 0.256 (see Table 2). Δ(BC) is plotted as a function of the surface gravity, that decreases steadily from the lower main sequence to the tip of the RGB. We also label the evolutionary phases corresponding to the selected isochrone points: main sequence (MS), turn off (TO), subgiant branch (SGB), and RGB. Only filters showing the largest positive and negative differences are displayed (see text for details).