  The detonation of a helium shell on top of a carbon-oxygen white dwarf has
been argued as a potential explosion mechanism for type Ia supernovae (SNe~Ia).
The ash produced during helium shell burning can lead to light curves and
spectra that are inconsistent with normal SNe~Ia, but may be viable for some
objects showing a light curve bump within the days following explosion. We
present a series of radiative transfer models designed to mimic predictions
from double detonation explosion models. We consider a range of core and shell
masses, and systematically explore multiple post-explosion compositions for the
helium shell. We find that a variety of luminosities and timescales for early
light curve bumps result from those models with shells containing $^{56}$Ni,
$^{52}$Fe, or $^{48}$Cr. Comparing our models to SNe~Ia with light curve bumps,
we find that these models can reproduce the shapes of almost all of the bumps
observed, but only those objects with red colours around maximum light ($B-V
\gtrsim 1$) are well matched throughout their evolution. Consistent with
previous works, we also show that those models in which the shell does not
contain iron-group elements provide good agreement with normal SNe~Ia of
different luminosities from shortly after explosion up to maximum light. While
our models do not amount to positive evidence in favour of the double
detonation scenario, we show that provided the helium shell ash does not
contain iron-group elements, it may be viable for a wide range of normal
SNe~Ia.
