Ratio of the stellar fluxes in mid-IR dust-dominated bands (from top to bottom: IRAC 8 μm, WISE 12 μm, WISE 22 μm, and MIPS 24 μm) and NIR dust-free bands (from left to right: Ks 2.2 μm, WISE 3.4 μm, and IRAC 3.6 μm) bands depending on the age of the galaxy (x-axis) and the τ constant of a “delayed” SFH (y-axis). The colour indicates the value of the ratio of the fluxes according to the colour bar to the right of each plot, with white lines indicating isocontours. A total of 22 500 models were computed representing the combination of 150 parameters on each axis. The Bruzual & Charlot (2003) models were adopted, assuming a Salpeter (1955) IMF and a metallicity Z = 0.02.