Normalized flux differences from the mean for six different models, for four different wavelength regions. The six model codes appear across the bottom of panel a) in the colours used in the plots; a) blue region with relatively low rms difference; b) region containing Hα; c) region containing the IR Ca II triplet 8498 Å line; d) region with high rms difference due to differences in a strong Mg I line at 8807 Å.