Models computed by CIGALE for the [ versus ] colours. Left: when no fine structure emission lines are added to the modelled spectra, the modelled galaxies are mainly located on a prime sequence extending from the bottom right to the top left. However, a clump of high-redshift observed galaxies, with colours [ versus ] ≈ [−0.1, −0.1], and with a total offset from the main sequence of the order of Δ_colour ∼ 0.1 − 0.2 cannot be reached by these models without emission lines. Right: emission lines are added to the continuum. When large equivalent width fine structure lines (and more specifically, EW([CII]157.6 μm) ∼ 10–20 μm) are added (Appendix A), the CIGALE models cover this clump of high-redshift galaxies. The three highest points are only partially covered by the models. Even though the two top ones (namely SPT0243-49 and SPT0245-63) are clearly high-redshift objects, respectively at z = 5.702 and z = 5.626, we assumed that the partial coverage is not sufficient and do not use these two objects in the rest of the paper. For the third one (SPT0348-62 at z = 5.654), the coverage by the models allows us to keep it in the analysis. We also identify SPT0418-47, for which we have five emission lines in the considered wavelength range, and an amplification factor of μ = 32.70 (Reuter et al. 2020).