Upper and middle panels: Observed photometry in LSST for three detected MBHBs (upper panels) and the stellar component of their host galaxies (middle panels). Blue, green, red, purple, and brown dots correspond to the flux inside the u, g, r, i, z filters, respectively. The properties of the binary and the host galaxy are indicated in each panel. Lower panels: Observed SED ($\nu L_{\nu }^{\mathrm{obs}}$) of the MBHB used to compute the photometry (ν and λ are the observed frequency and wavelength, respectively). The blue, orange and purple lines correspond to the emission of the primary MBH, secondary MBH and circumbinary disk, respectively; while the thick gray line is the sum of the three components. The vertical shaded area corresponds to the optical range covered by LSST.