Relative brightness of 67P dust normalised at 5500 Å. (a) The dust colour slopes (Eq. (14)) are shown as the solid lines with 3σ fitting errors in grey regions. (b) Two solid black lines indicate the lower (5 ± 2% (1000 Å)^-1) and upper (18 ± 2% (1000 Å)^-1) reflectivity limits of the dust of nine comets over 3500–6500 Å, respectively (Jewitt & Meech 1986). The thick grey solid and dashed lines indicate the average reflectivity trend and standard deviation of 31 active comets (8.4 ± 3.6% (1000 Å)^-1) over 3540–9050 Å used in Solontoi et al. (2012).