WISE color–color diagram, similar to the one presented in Gómez et al. (2017), including all known and new maser-emitting PNe and candidates, and adopted from Cala et al. (2022). The stars and squares represent the confirmed maser-emitting PNe and candidates, respectively, as presented in Fig. 4. The red color stands for WF nature of the source. The open circles represent obscured post-AGB candidates (Ramos-Larios et al. 2009, 2012). The solid line represents the loci of the colors for blackbody brightness distributions. The tick marks in the blackbody line go from 150 to 350 K at increment steps of 25 K.