Context : Shells in Elliptical Galaxies are faint , sharp-edged features , believed to provide evidence for a merger event .
Accurate photometry at high spatial resolution is needed to learn on presence of inner shells , population properties of shells , and dust in shell galaxies .

Aims : Learn more about the origin of shells and dust in early type galaxies .

Methods : V-I colours of shells and underlying galaxies are derived , using HST Advanced Camera for Surveys ( ACS ) data .
A galaxy model is made locally in wedges and subtracted to determine shell profiles and colours .
We applied Voronoi binning to our data to get smoothed colour maps of the galaxies .
Comparison with N-body simulations from the literature gives more insight to the origin of the shell features .
Shell positions and dust characteristics are inferred from model galaxy subtracted images .

Results : The ACS images reveal shells well within the effective radius in some galaxies ( at 0.24 r _ { \mathrm { e } } = 1.7 kpc in the case of NGC 5982 ) .
In some cases , strong nuclear dust patches prevent detection of inner shells .
Most shells have colours which are similar to the underlying galaxy .
Some inner shells are redder than the galaxy .
All six shell galaxies show out of dynamical equilibrium dust features , like lanes or patches , in their central regions .
Our detection rate for dust in the shell ellipticals is greater than that found from HST archive data for a sample of normal early-type galaxies , at the 95 % confidence level .

Conclusions : The merger model describes better the shell distributions and morphologies than the interaction model .
Red shell colours are most likely due to the presence of dust and/or older stellar populations .
The high prevalence and out of dynamical equilibrium morphologies of the central dust features point towards external influences being responsible for visible dust features in early type shell galaxies .
Inner shells are able to manifest themselves in relatively old shell systems .