Parameters of the reddening distribution (see Eq. (7)) obtained from fitting the observed SN colours conditioned on the Gaussianity of the intrinsic colour distribution. The results shown as 68 and 95 per cent credibility contours are compared to the predictions of the dust model from this study. The apparent discrepancy between the observational constraints and the dust model for ET host galaxies implies that either the observed red colours are primarily driven by a larger variation in SN intrinsic colours or ET host galaxies are unusually rich in interstellar dust. The square and triangle symbols show the effect of increasing the dust-to-stellar mass ratio by 1.0 dex and 1.5 dex.