Mid-IR colour-colour plot. Black and red crosses correspond to star-forming and AGN SEDs respectively. The wedge defines the AGN region following the selection criteria of Stern et al. (2005). The colour tracks define two different template SEDs as a function of redshift up to z = 3. The blue line represents the Mrk231 (ULIRG, Compton-thick) template, while the green line represents Arp220 (ULIRG, star-forming) SED. For each template the position of the redshift z = 0 is marked with a large cross. The templates are taken from Polletta et al. (2007).