Multi-epoch spectra for model r0e2 (left panel) and model r0e2n (right panel). The quantity λF_λ is shown to better reveal the fainter flux at longer wavelength. In the left panel, the thin line corresponds to the continuum flux (the offset with the full spectrum arises from the contribution from lines). What drives the difference in spectral and color properties between the two models is magnetar heating since the same ejecta (composition, mass, kinetic energy) is used for both (see discussion in Sect. 3).