  As Type Ia supernova cosmology transitions from a statistics dominated to a
systematics dominated era, it is crucial to understand leftover unexplained
uncertainties affecting their luminosity, such as the ones stemming from
astrophysical biases. Indeed, SNe Ia are standardisable candles, whose absolute
magnitude reach a 0.15~mag scatter once empirical correlations with their
lightcurve stretch and colour and with their environment are accounted for. In
this paper, we investigate how the standardisation process of SNe Ia depends on
environment, to ultimately reduce their scatter in magnitude, focusing on
colour standardisation. We use the volume-limited ZTF SN Ia DR2 sample, which
offers unprecedented statistics for the low redshift ($z<0.06$) range. We first
study the colour distribution, focusing on the effects of dust, to then select
a dustless subsample of objects from low stellar mass environments and from the
outskirts of their host galaxies. We then look at the colour-residuals relation
and its associated parameter $\beta$. Finally, we investigate the colour
dependency of the environment-dependent magnitude offsets (steps), to try to
disentangle intrinsic and extrinsic colour origin. Our sample probes well the
red tail of the colour distribution, up to $c=0.8$. The dustless sample
exhibits a significantly lower red tail ($4.6\sigma$) in comparison to the
whole sample. This suggests that reddening above $c\geq0.2$ is dominated by
host interstellar dust absorption. Looking at the colour-residuals relation, we
find it to be linear with lightcurve colour. We show hints of a potential
evolution of $\beta$ with host stellar mass at a $2.5\sigma$ level. Finally,
unlike recent claims from the literature, we see no evolution of steps as a
function of lightcurve colour, suggesting that dust may not be the dominating
mechanism responsible for the environmental dependency of SNe Ia magnitude.
