  The colour evolution of reddened Type Ia supernovae can place strong
constraints on the location of dust and help address the question of whether
the observed extinction stems from the interstellar medium or from
circumstellar material surrounding the progenitor. Here we analyse BV
photometry of 48 reddened Type Ia supernovae from the literature and estimate
the dust location from their $B-V$ colour evolution. We find a time-variable
colour excess $E(B-V)$ for 15 supernovae in our sample and constrain dust to
distances between 0.013 and 45 pc ($4\times10^{16}-10^{20}$ cm). For the
remaining supernovae, we obtain a constant $E(B-V)$ evolution and place lower
limits on the dust distance from the explosion. In all the 48 supernovae, the
inferred dust location is compatible with an interstellar origin for the
extinction. This is corroborated by the observation that supernovae with
relatively nearby dust ($\lesssim$ 1 pc) are located close to the center of
their host galaxy, in high-density dusty regions where interactions between the
supernova radiation and interstellar clouds close by are likely to occur. For
supernovae showing time-variable $E(B-V)$, we identify a potential preference
for low $R_\mathrm{V}$ values, unusually strong sodium absorption and
blue-shifted and time-variable absorption features. Within the interstellar
framework, this brings evidence to a proposed scenario where cloud-cloud
collisions induced by the supernova radiation pressure can shift the grain size
distribution to smaller values and enhance the abundance of sodium in the
gaseous phase.
