  The colour-magnitude relation (CMR) of cluster ellipticals has been widely
used to constrain their star formation histories (SFHs) and to discriminate
between the monolithic and merger paradigms of elliptical galaxy formation. We
investigate the elliptical CMR predicted in the merger paradigm by using a LCDM
hierarchical merger model. We first highlight sections of the literature which
indicate that the traditional use of fixed apertures to derive colours gives a
distorted view of the CMR due to the presence of colour gradients in galaxies.
Fixed aperture observations make the CMR steeper and tighter than it really is.
We then show that the star formation history (SFH) of cluster ellipticals
predicted by the model is quasi-monolithic, with over 95 percent of the total
stellar mass formed before a redshift of 1. The quasi-monolithic SFH produces a
predicted CMR that agrees well at all redshifts with its observed counterpart
once the fixed aperture effect is removed. More importantly, we present
arguments to show that the elliptical-only CMR can be used to constrain the
SFHs of present-day cluster ellipticals only if we believe a priori in the
monolithic collapse model. It is not a meaningful tool for constraining the SFH
in the merger paradigm, because a progressively larger fraction of the
progenitor set of present-day cluster ellipticals is contained in late-type
star forming systems at higher redshift, which cannot be ignored when deriving
the SFHs. Hence, the elliptical-only CMR is not a useful discriminant between
the two competing theories of elliptical galaxy evolution.
