Evolution of the predicted colours with redshift. When compared to models without lines (crosses), at log₁₀U = −4.0 (upward triangles), the strong [CII]157.6 μm line induces an upward move of log₁₀ that corresponds to the clump of galaxies at z ∼ 4–6. However, even though the move of the [OIII]51.8 μm and [OIII]88.3 μm lines of galaxies with a strong emission from HII regions at log₁₀U = −2.0 (downward triangles) could induce specific colours, the effect is less clear as models with no lines can also lie in this same region of the plot (Fig. 3). All the symbols are colour-coded by redshift.