Left panel: ABC sources with a WISE counterpart detected in all four WISE bands (white circles) represented in the w1 − w2 versus w2 − w3 color–color plane. The same KDE isodensity contours of Fig. 20 are presented with lines of different colors for the different Roma-BZCAT subclasses, as indicated in the legend. Filled red and black circles mark sources with an optical spectroscopic classification of QSO and GALAXY, respectively. Right panel: same as the left panel, but for the w2 − w3 versus w3 − w4 projection.