Upper panels: colours of the 41 696 SDSS QSOs whose spectra were searched for C i absorption versus QSO emission redshift. (g−r), (r−i) and (r−z) colours are displayed in the left-, middle-, and right-hand panels, respectively. The thick blue line shows the median QSO colour as a function of redshift. Thin blue lines show one standard deviation away from that median. The red circles indicate the 66 lines of sight passing through C i-detected gas. Lower panels: distribution of colour excesses (defined as, e.g.: (g−r)−⟨g−r⟩_zQSO for Δ(g−r)) for the 66 QSOs whose spectra were found to exhibit C i absorption (thick red histogram) compared to the whole QSO sample (thin-hashed histogram, scaled down by a factor of 632 to have the same area). In each panel (from left to right: Δ(g−r), Δ(r−i) and Δ(r−z)), vertical dotted red lines indicate the median colour excesses of the sample of QSOs with detected C i absorption. The median values for the whole QSO sample are shown by vertical dashed lines.