U−B vs. Q color–color diagrams of the associations, using the same color code as in Fig. A.1. Theoretical isochrones are included for reference. Since no extinction correction was applied, the lines mark the locus of an unreddened population. A population parallel to the isochrones but shifted indicates reddening. Note that the isochrones of all ages overlap. This provides further supporting evidence for the selection criteria (Q <  −0.4) to find candidate blue massive stars used in Garcia et al. (2009).