Upper left: IRAC1 vs. IRAC1–IRAC2 diagram. The RSG sample mainly shows negative value of IRAC1–IRAC2 due to the CO absorption around 4.6 μm. In particular, it turns bluewards (IRAC1 ≳ 9.5 mag) with CO absorption, redwards (8.5 ≲ IRAC1 ≲ 9.5) with enhanced mass loss due to both luminosity and variability, and again bluewards (IRAC1 ≲ 8.5) with PAH emission, along with the increasing of luminosity. There is another fainter branch (9.3 ≲ IRAC1 ≲ 10.0 mag and 0 ≲ IRAC1 − IRAC2 ≲ 0.35 mag) extended to the x-AGBs region with unknown origin. Upper right: IRAC4 vs. J − IRAC4 diagram. The dash-dotted lines region indicates the selection criteria for the obscured target. Bottom left: WISE3 vs. K_S − WISE3 diagram. For clarity, RSG candidates with S/N_WISE3 <  10 are shown as red open circles, while background targets from SMC source catalog with S/N_WISE3 <  10 are not shown in the diagram. The RSGs population shows redder color of J − IRAC4 and K_S − WISE3 along with the increasing of IRAC4/WISE3 luminosity. Bottom right: MIPS24 vs. K_S − MIPS24 diagram. The majority of the MIPS24 detected targets are also spectroscopic RSGs. The brightest RSGs candidates form a parallel sequence with respect to the x-AGBs population. The vertical dashed lines indicate the approximate blue boundaries of x-AGBs. See text for details.