Recent analyses have found intriguing correlations between the colour (c) of type Ia supernovae (SNe Ia) and the size of their ‘mass- step’, the relationship between SN Ia host galaxy stellar mass (M_stellar) and SN Ia Hubble residual, and suggest that the cause of this relationship is dust. Using 675 photometrically classified SNe Ia from the Dark Energy Survey 5-yr sample, we study the differences in Hubble residual for a variety of global host galaxy and local environmental properties for SN Ia subsamples split by their colour. We find a 3ensuremathsigma difference in the mass-step when comparing blue (c < 0) and red (c > 0) SNe. We observe the lowest r.m.s. scatter (raisebox-0.5extextasciitilde0.14 mag) in the Hubble residual for blue SNe in low mass/blue environments, suggesting that this is the most homogeneous sample for cosmological analyses. By fitting for c-dependent relationships between Hubble residuals and M_stellar, approximating existing dust models, we remove the mass-step from the data and find tentative raisebox-0.5extextasciitilde2ensuremathsigma residual steps in rest-frame galaxy U - R colour. This indicates that dust modelling based on M_stellar may not fully explain the remaining dispersion in SN Ia luminosity. Instead, accounting for a c-dependent relationship between Hubble residuals and global U - R, results in ensuremathleq1ensuremathsigma residual steps in M_stellar and local U - R, suggesting that U - R provides different information about the environment of SNe Ia compared to M_stellar, and motivating the inclusion of galaxy U - R colour in SN Ia distance bias correction.