Left panel: Spectral energy distribution (SED) of two selected MBHBs from our lightcone. The dashed, dotted and dash-dotted lines correspond to $L_{\nu }^{\mathrm{mini},1}$, $L_{\nu }^{\mathrm{mini},2}$ and $L_{\nu }^{\mathrm{CBD}}$, respectively. The solid lines correspond to the total SED (L_ν). Right panel: Apparent magnitudes in the LSST bands (m_LSST) associated with the SEDs of the left panels (coloured dots). The vertical lines locate the LSST filter central wavelengths. The horizontal coloured ticks indicate the filter sensitivities, while the grey dashed lines highlight the non-detectable magnitudes.