B−I (top) and B−V (bottom) colour curves of SN 2005la, compared with those of a few Type Ibn SNe. We remark that the B−I colour is less affected by the flux contamination of the narrow He i circumstellar lines than the B−V colour. Differences in the intensity of the He i circumstellar lines may explain why SNe Ibn show a very similar B−I colour evolution, but, simultaneously, rather different B−V colours. The amount of interstellar extinction adopted for the SNe Ibn sample is the same as in Table 1 of Pastorello et al. (2008).