  The effects of late gas accretion episodes and subsequent merger-induced
starbursts on the photo-chemical evolution of elliptical galaxies are studied
and compared to the picture of galaxy formation occurring at high redshift with
an unique and intense starburst modulated by a very short infall, as suggested
by Pipino & Matteucci (2004, Paper I). By means of the comparison with the the
colour-magnitude relations and the [<Mg/Fe>_V]-sigma relation observed in
ellipticals, we conclude that either bursts involving a gas mass comparable to
the mass already transformed into stars during the first episode of star
formation and occurring at any redshift, or bursts occurring at low redshift
(i.e. z<0.2) and with a large range of accreted mass, are ruled out. These
models fail in matching the above relations even if the initial infall
hypothesis is relaxed, and the galaxies form either by means of more
complicated star formation histories or by means of the classical monolithic
model. On the other hand, galaxies accreting a small amount of gas at high
redshift (i.e. z>3) produce a spread in the model results, with respect to
Paper I best model, which is consistent with the observational scatter of the
color-magnitude relations, although there is only marginal agreement with the
[<Mg/Fe>_V]-sigma relation. Therefore, only small perturbations to the standard
scenario seem to be allowed. We stress that the strongest constraints to galaxy
formation mechanisms are represented by the chemical abundances, whereas the
colours can be reproduced under several different hypotheses.
