The relationship between the SPIRE colour–colour indices S250/S350 vs. S350/S500 for galaxies coded according to their different physical parameters as in Fig. 4. The colour–colour relation is compared to the expected relations obtained for a modified black body with a grain emissivity index β = 2 (solid line) and β = 1.5 (dotted line). Black squares indicate different temperatures for the two modified black bodies (left panel). The Spearman correlation coefficient of this relation is r = 0.90.