Upper panel: the S60/S250 vs. S60/S100 colour–colour relation with galaxies coded according to their physical parameters (from left to right) as a function of the metallicity (red crosses for 12 + log(O/H)  ≤  8.55, blue circles for 12 + log (O/H)  >  8.55), Hα surface brightness (grey circles for log Σ(Hα)  ≤  38.6 erg s^-1 kpc^-2, green crosses for log Σ(Hα)  >  38.6 erg s^-1 kpc^-2), birthrate parameter (magenta three points stars for log b  ≤  0.2, cyan squares for log b > 0.2), H band effective surface brightness (yellow stars for μ_e(H) ≤ 19 AB mag arcsec^-2, grey triangles for μ_e(H) > 19  AB mag arcsec^-2) and UV attenuation (magenta circles for A(FUV) < 1.4 mag, green stars for A(FUV) ≥ 1.4). Middle panel: the dependence of the position of galaxies along the S60/S250 vs. S60/S100 colour–colour relation (X_∥) on the different physical parameters. Lower panel: the histogram of the distribution of galaxies along the colour–colour relation (upper) and the relation between the position of galaxies in the direction perpendicular (Y_⊥) and that along (X_∥) the colour–colour relation. r gives the Spearman correlation coefficient of the different relations, KS the probability that the two galaxy populations are driven by the same parent population (Kolmogorov-Smirnov test: for KS  ≤  5 the two galaxy populations are statistically different). The de-projection of the S60/S100 vs. S60/S250 colour–colour relation is done using the measured linear best fit: S60/S250 = 2.64( ± 0.16)  ×  S60/S100 – 0.45(±0.06); r = 0.83.