Left panels: top figure: procedure of spectral composition in our MC simulations for an example case. The composite spectrum is the sum of a SN template (here a SN Ia at +10d), a BB continuum (here R_BB = 3.2 × 10¹⁵ cm and T_BB = 8000 K), and an emission line spectrum. Bottom panel: we reddened the composite spectrum (by a total A_V = 0.66 mag) and added noise (here S/N = 40) to obtain the final spectrum in the (maximum) wavelength range 3000−9000 Å. All fluxes are shown in the absolute scale. Middle panels: the black-body radii and temperatures that were used in the MC simulation as a function of phase. Bottom panel: histogram of the resulting distribution of absolute magnitudes M_V. The obtained values range from −22.5 to −16 and nicely overlap with the observed dispersion in SN IIn luminosities. Right panel: more example spectra taken from the final MC simulation and spanning a wide distribution of underlying SN types, phases, BB continuum strength, total extinction, and S/N. The values for these parameters are noted next to the spectra. The CSM contamination affects the spectral appearance. The spectra are shown in a logarithmic scale and have been shifted by arbitrary constants for presentation purposes.