Example g′−r′ colour maps for our dust classification. The tick labels along the colour-bar indicate median−4σ, median−2σ, median, and median+2σ, respectively. The colour bars are limited between median−5σ and median+3σ. The median and standard deviation values of the g′−r′ colour are calculated in the region of 0−1 R_eff, which is shown with the black ellipse. The white scale bar at the bottom corner indicates 1 kpc. On the top-right, we show an illustration of the position angles of the various galaxy components: in black the optical-disc from Krajnović et al. (2011); in purple the H I-disc from Serra et al. (2014); and in red the CO-disc from Alatalo et al. (2013). Similar g′−r′ colour maps of all of the galaxies studied here are shown in Appendix C.