Colour–colour diagrams (left panels: r−i vs. g−r; right: r−z vs. g−i) of SDSS LRG in two redshift bins (top: 0.15 < z < 0.25; bottom: 0.25 < z < 0.35). The grey colour map accounts for the density of sources. Galaxy colours are derived from extinction-corrected griz model magnitudes (see text for details). The coloured small-dotted tracks show the colours of two template libraries from Greisel et al. (2013) for LRG at z ~ 0.2 and z ~ 0.3 (top: z = [0.15,0.25]; bottom: z = [0.25,0.35]; bin = 0.01; purple to red shows increasing redshift). The two optimal templates at z ~ 0.2 and z ~ 0.3 adopted as reference template for the calibration of the dust-to-reddening factor are shown by the larger dotted tracks.