  The properties of low-redshift Type Ia supernovae are investigated using
published multi-band optical broadband data from the Calan/Tololo and CfA
surveys. The average time evolution of B-V, V-R, R-I, B-I and V-I, the
intrinsic dispersion and time correlations are studied. This information is
required to deduce the extinction of such explosions from the measured colours.
We find that extinction corrections on individual SNe based on their colours up
to 40 days past the B-band lightcurve maximum are generaly limited to
\sigma_{A_V} \gsim 0.1, due to intrinsic variations, as far as it can be
conservatively deduced with the current sample of data. However, we find that
the V-R colour, especially at late times, is consistent with a negligible
intrinsic spread, and may be the most accurate estimator for extinction.
