Redshift evolution of the mid-to-far infrared colours of Herschel galaxies. Each plot shows the ratio of the observed Herschel over MIPS-24 μm flux densities as a function of redshift. Figures on the left column show the colours from the observed Herschel measurements. Figures on the right column show the same dataset but after having corrected the 160 to 500 μm over 24 μm ratios of the individual galaxies by the offset observed in the 100 to 350 μm shorter wavelength bands. The points indicate the positions of individually detected sources separated in 3 populations, depending on the presence of bright 24 μm neighbours (i.e. with a S_{24 μm} greater than half of the central source) in their close environment (at a distance of 12′′for PACS and 20′′for SPIRE). This number is the “clean index”, CI. Filled red dots: galaxies with no bright neighbours (CI = 0); filled green dots: galaxies with at most one bright neighbour (CI ≤ 1); open circles: remaining 3σ sources (CI ≥ 2). The sliding median of the CI = 0 galaxies is shown with a blue line and blue circles with central red dots. The trends for the mean stacking at the positions of the 24 μm prior sources brighter than 50 μJy are illustrated with orange lines. The yellow line corresponds to the best-fit relations described in Eq. (1) to (5) of Sect. 5 that fit the sliding median. The colours obtained for the IR spectral energy distributions of the library of Magdis et al. (2012, black dotted line) and the main sequence SED of Elbaz et al. (2011) are shown for comparison.