  An excess of flux (i.e. a bump) in the early light curves of type Ia
supernovae has been observed in a handful of cases. Multiple scenarios have
been 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 amount
of $^{56}$Ni in the outer ejecta ($\sim$0.03~$M_{\rm{\odot}}$). We present a
series of model light curves and spectra for ejecta profiles containing
$^{56}$Ni shells of varying masses (0.01, 0.02, 0.03, and
0.04~$M_{\rm{\odot}}$) and widths. We find that even for our lowest mass
$^{56}$Ni shell, an increase of \textgreater2 magnitudes is produced in the
bolometric 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 $^{56}$Ni
shell differs significantly from those without and shows a colour inversion
similar to some double-detonation explosions. Spectra of our $^{56}$Ni shell
models show that strong suppression of flux between $\sim$3\,700 --
4\,000~$\AA$ close to maximum light appears to be a generic feature for this
class 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 match
shapes of the early optical light curve bumps, but the colour and spectral
evolution are in disagreement. This would indicate that an alternative origin
for the flux excess is necessary. Based on existing explosion scenarios,
producing such a $^{56}$Ni shell in the outer ejecta as required to match the
light curve shape, without the presence of additional short-lived radioactive
material, may prove challenging. Given that only a small amount of $^{56}$Ni in
the outer ejecta is required to produce a bump in the light curve, such
non-monotonically decreasing $^{56}$Ni distributions in the outer ejecta must
be rare, if they were to occur at all.
