Sample observed colour–colour diagram with different symbols indicating age ranges: t(Myr) < 7 (blue circle), 7 < t(Myr) < 20 (green square), 20 < t(Myr) < 100 (red diamond). The three young clusters near the Galactic centre and M 42 are labelled. Continuous and dashed lines represent the colours of the Padova SSPs and the SB99 CSP, respectively. Model ages range from very early phases to 100 Myr, indicated for the CSP models. The magenta dot-dashed line connects the observed and the intrinsic colours of Quintuplet. Blue dotted lines show the extinction yielded by A_V = 5 for both a standard screen reddening vector and a dusty cluster medium.