This figure identifies the emission lines that produce excesses in the broad-bands when the redshift increases. Top: At log₁₀ U = -4.0, the strong [CII]157.6 μm lines enter the LABOCA_870μm filters at z ∼ 4 and exit at z ∼ 5.2. The line induces an upward move of the corresponding colour. [SIII]34.8 μm enters the PSW_250μm at z ∼ = 5.5, which boosts the colour. We find a total move of the galaxies by about Δ_total ∼ 0.10 in the colour-colour diagram. These two combined effects explain that PDR-dominated galaxies at 4 ≲ z ≲ 6 are offset from the other galaxies. Bottom: Movement of the [OIII]51.8 μm and [OIII]88.3 μm lines of galaxies with a strong emission from HII regions at log₁₀ U = -2.0 could induce specific colours that could help identify such galaxies from this colour-colour diagram. However, the effect is less clear as models with no lines can also lie here (Fig. 3).