  We present an unbiased method for evaluating the ranges of ages and
metallicities which are allowed by the photometric properties of the stellar
populations that dominate the light of early-type galaxies in clusters. The
method is based on the analysis of morphologically-classified early-type
galaxies in 17 clusters at redshifts 0.3<z<0.9 and in the nearby Coma cluster
using recent stellar population synthesis models that span a wide range of
metallicities. We confirm that metallicity effects must play a role in the
origin of the slope of the color-magnitude relation for cluster early-type
galaxies. We show, however, that the small scatter of the color-magnitude
relation out to redshifts z=1 does not formally imply a common epoch of major
star formation for all early-type galaxies. Instead, it requires that galaxies
assembling more recently be on average more metal-rich than older galaxies of
similar luminosity. Regardless of the true ages and metallicities of early-type
galaxies within the allowed range, their photometric properties and the implied
strengths of several commonly used spectral indices are found to be consistent
with apparently passive evolution of the stellar populations. Also, the implied
dependence of the mass-to-light ratio on galaxy luminosity is consistent with
the observed trend. The results of our unbiased analysis define the boundaries
in age and metallicity that must be satisfied by theoretical studies aimed at
explaining the formation and evolution of early-type galaxies in clusters.
