Comparison between the distribution of supernova colour parameters in the calibration sample and the best-fit model with the reddening scale, τ, measured either from the Hubble flow supernovae (model A) or from the calibration supernovae (model B). The best-fit reddening scale in the calibration sample is larger than in the Hubble flow, and this preference is driven by a higher fraction of reddened supernovae (c ∼ 0.1) than in the corresponding young population from the Hubble flow.