Top: NUV−i colour of SDSS galaxies in the Voyer et al. (2014) catalogue as a function of the Galactic dust reddening (taking only objects with signal-to-noise higher than 10). The solid lines show the average, while the dashed lines indicate the 1-sigma dispersion. The dust reddening is derived either from the Schlegel maps (as given in the GUViCS catalogue), from the Planck E(B−V) map, or from our FUV 1 arcmin or 20 arcsec resolution diffuse emission maps. The arrow indicates the expected reddening trend (see Sect. 3.6). Bottom: the 1-sigma dispersion of the NUV−i colour is indicated as a function of the reddening. The range plotted in both panels corresponds to the bins where we do obtain E(B−V) values for each method.