  Type Ia Supernovae (SNe Ia) spectra are compared regarding the coefficient of
the largest wavelet scale in their decomposition. Two distinct subgroups were
identified and their occurrence is discussed in light of use of SNe Ia as
cosmological probes. Apart from the group of normal SNe, another trend
characterised by intrinsically redder colours is consisted of many different SN
events that exhibit diverse properties, including the interaction with the
circumstellar material, the existence of specific shell-structure in or
surrounding the SN ejecta or super-Chandrasekhar mass progenitors. Compared
with the normal objects, these SNe may violate the standard width-luminosity
correction, which could influence the cosmological results if they were all
calibrated equally, since their fraction among SNe Ia is not negligible when
performing precision cosmology. Using largest wavelet scale coefficient in
combination with long-baseline B-I colours, we show how to disentangle SN
intrinsic colour from the part that corresponds to the reddening due to dust
extinction in the host galaxy in the SALT2 colour parameter c, discussing how
the intrinsic colour differences may explain the different reddening laws for
two subsamples. There are wavelength intervals for which the measured largest
scale coefficient is invariant to the additional extinction applied to a
spectrum. Combination of wavelet coefficients measured in different wavelength
intervals can be used to develop a technique that allows for estimation of
extinction.
