Herschel/SPIRE colour-colour plot (S_500 μm/S_250 μm vs. S_500 μm/S_350 μm) for the z-GAL and Pilot Programme sources (shown as blue squares). Typical uncertainties on the colours are shown in the bottom right corner of the figure. The sources listed in Bakx et al. (2018) and Nayyeri et al. (2016) with spectroscopic redshifts (labelled ‘z-spec’ and listed in Table A.5) are included for comparison and indicated with a red star. The sources that were excluded from the z-GAL selection (labelled “Excluded”) are indicated with a red dot (see Table A.4). The side histograms display the distribution of the values as a function of the number of sources. The overlaid curve shows the redshift track expected for a galaxy with a spectrum energy distribution comparable to the Cosmic Eyelash galaxy; the photometric redshifts are indicated along the curve (see Ivison et al. 2016).