Radial evolution of the spectral slope of the dust particles of 67P in % (1000 Å)^-1. Circles indicate the average value of our three-epoch colour data (Fig. 9a). Blue and red symbols indicate spectral slopes measured at wavelengths shortward and longward of the inflection point. Horizontal error bars of triangles span the cometocentric distance (1.0–2.5 km) VIRTIS covered, while their vertical error bars denote the error of the weighted mean of observations (i.e. the standard deviation divided by the square root of the number of the data points) between UT2016 January and early March.