Residuals of colour index Herschel band minus MIPS-24 μm. with respect to the best-fit relations as a function of redshift. Upper panel: residuals for the observed colour index. The 68% dispersion around the median is equal to [0.17, 0.16, 0.18, 0.21, 0.31] dex for the five Herschel bandpasses when accounting for redshift ranges in the best-fit equations, while it would have been [0.20, 0.18, 0.20, 0.33, 0.42] without separating galaxies in redshift slices. Lower panel: dispersion after accounting for the offset measured at shorter wavelengths, e.g. correcting S_{160 μm}/S_{24 μm} by the distance of S_{100 μm}/S_{24 μm} to the median, the dispersion in colour indices becomes as small as [0.14, 0.12, 0.13, 0.19] at [160, 250, 350, 500] μm, respectively (no correction can be applied to the 100 μm band obviously). The correction process reduces the dispersion of the colour distribution because that variation is physical and not due to noise in the measurements.