Q2 parameter versus the GLIMPSE [3.6]–[4.5] color. Symbols are as in Fig. 1. The arrow represents the reddening vector following the extinction ratios given by Indebetouw et al. (2005). Dashed vertical lines at Q2 =  −1.1 mag and Q2 = 1.37 mag indicate the region populated by RSG stars. Stars in this range and with  [3.6]  −  [4.5]  <  − 0.4 mag are highly probable RSG stars. Typical errors are 0.13 mag in the x-axis and 0.08 mag in the y-axis.