Distribution of (a) NIRCam colours, (b) MIRI colours and (c) F444W AB magnitude as a function of redshift, with lines of constant stellar mass overlaid in the latter. A representative error bar is indicated in the lower right corner of each panel. Panel a) demonstrates the SMGs are much redder than our massive and sSFR-matched field populations, as was expected. Panel b) highlights that a fraction of SMGs and field galaxies may host AGNs with a very red F770W−F1800W colour and bluer F444W−F770W, indicating a strong upturn in the SED beyond rest-frame 3 μm. We label the ‘AGNs’ and SFG regions in the colour space adapted from Kirkpatrick et al. (2013, 2017). We note the F770W−F1800W > 1 colour can also be driven by the 6.2 μm PAH feature detected in the F18000W filter at z ∼ 1.9. Panel c) demonstrates that the SMGs are the most massive (and so typically also brightest) galaxies at their epoch where the two field samples have been selected to be the most massive low far-infrared luminosity galaxies.