  The current formation models for cluster elliptical galaxies which
incorporate a mechanism for the metallicity enhancement of massive ellipticals
predict a change in the observed slope of the red sequence of ellipticals as a
function of redshift. This change occurs primarily because the metal-rich
galaxies become redder faster than the metal-poorer galaxies with increasing
age. This effect is most pronounced within ~ 4 Gyr of formation. Observations
of the change of the slope of the red sequence with redshift may thus be used
to constrain the formation epoch for galaxy clusters. We examine the red
sequence of cluster ellipticals using publicly available HST imaging data for a
set of six 0.75>z>0.2 clusters, and a sample of 44 Abell clusters at z<0.15
imaged with the KPNO 0.9 m. We compare the derived slopes of the red sequences
with a set of cluster-elliptical evolution models and find good agreement. We
demonstrate that such a comparison provides a useful constraint on the
formation epoch for clusters, which can be made independently from
considerations of absolute color evolution and scatter in the red sequence.
  From our initial comparison of the observed and model slopes as a function of
redshift, we conclude, conservatively, that most of the elliptical galaxies in
the cores of clusters must form at z>2.0, and that these galaxies are coeval
and passively evolving.
