Multi-wavelength pixel colours of SMGs and field galaxies. a) Average F200W − F444W colour of the faintest pixels (below 80^th percentile of flux) inside 0$\stackrel{″}{.}$5 elliptical apertures in the F444W image for the SMGs and field samples, as a function of the average colour for the top 20 percent brightest F444W pixels. Both field samples exhibit almost uniform colours between faint and bright regions, with a small offset to the equality line of 0.12 ± 0.03 mag (in the sense that the brighter regions are redder). The SMGs are both generally redder in fainter regions and significantly redder (1.11 ± 0.09 mag) in their brightest regions. In panel b) we show the average F200W − F444W colour of the brightest 20% and faintest 80% pixels as a function of ALMA pixel brightness. On the right-hand axis we show the surface dust mass density derived using the 870 μm flux to dust mass relation from Dudzevičiūtė et al. (2020) at the median redshift of the sample (z  =  2.7). The brightest regions in the near-infrared (NIRCam F444W), which have the reddest F200W − F444W colour, correspond to the brightest regions in the rest-frame far-infrared (ALMA 870 μm).