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An excess of flux (i.e. a bump) in the early light curves of type Iasupernovae has been observed in a handful of cases. Multiple scenarios havebeen proposed to explain this. It has been shown that for at least one object(SN~2018oh) the excess emission observed could be the result of a large amountof $^{56}$Ni in the outer ejecta ($\sim$0.03~$M_{\rm{\odot}}$). We present aseries of model light curves and spectra for ejecta profiles containing$^{56}$Ni shells of varying masses (0.01, 0.02, 0.03, and0.04~$M_{\rm{\odot}}$) and widths. We find that even for our lowest mass$^{56}$Ni shell, an increase of \textgreater2 magnitudes is produced in thebolometric light curve at one day after explosion relative to models without a$^{56}$Ni shell. We show that the colour evolution of models with a $^{56}$Nishell differs significantly from those without and shows a colour inversionsimilar to some double-detonation explosions. Spectra of our $^{56}$Ni shellmodels show that strong suppression of flux between $\sim$3\,700 --4\,000~$\AA$ close to maximum light appears to be a generic feature for thisclass of model. Comparing our models to observations of SNe~2017cbv and 2018oh,we show that a $^{56}$Ni shell of 0.02 -- 0.04~$M_{\rm{\odot}}$ can matchshapes of the early optical light curve bumps, but the colour and spectralevolution are in disagreement. This would indicate that an alternative originfor the flux excess is necessary. Based on existing explosion scenarios,producing such a $^{56}$Ni shell in the outer ejecta as required to match thelight curve shape, without the presence of additional short-lived radioactivematerial, may prove challenging. Given that only a small amount of $^{56}$Ni inthe outer ejecta is required to produce a bump in the light curve, suchnon-monotonically decreasing $^{56}$Ni distributions in the outer ejecta mustbe rare, if they were to occur at all.

An investigation of $^{56}$Ni shells as the source of early light curve bumps in type Ia supernovae