Filled contour plot of the mean absolute differences in the colour BP − RP between the reference isochrone with [Fe/H] = 0.1 and ΔY/ΔZ = 1.8, and all other isochrones in the grid (for the FRANEC MARCS set). The reference model is identified by the black diamond. Solid and dashed black lines indicate regions where the mean absolute difference is below 0.007 mag and 0.02 mag, respectively.