Optical color–color diagrams for WIBRaLS2 (upper panel) and KDEBLLACS (bottom panel) sources in the footprint of SDSS. The total number of sources is 3407 for WIBRaLS2 and 2807 for KDEBLLACS, and are divided in WBZBs/KBZBs (blue stars), WBZQs (orange circles) and WMIXEDs (violet squares), according to their classification in WIBRaLS2 and KDEBLLACS. The red, green, and black lines represent, respectively, the 90% (thin line) and 50% (thick line) isodensity contours based on the distribution of 10 000 galaxies, 10 000 QSOs, and 10 000 stars with available spectra in SDSS DR15.