Colour−colour diagram: PACS/SPIRE diagram: F₁₀₀/F₂₅₀ versus F₂₅₀/F₅₀₀. The colour and symbol choices are the same as in Fig. 9 for both figures. Note that the most metal-poor galaxies (from 0.03 to 0.20 Z_⊙) are very faint and even not detected anymore at long wavelengths. We were only able to derive upper limits beyond 160 μm for these galaxies and, thus, some galaxies do not appear on this diagram anymore.