  We analyse the cluster color-magnitude relation (CMR) for early-type galaxies
in two of the richer clusters in the z ~ 0.9 supercluster system to derive
average ages and formation redshifts for the early-type galaxy population. Both
clusters were observed with the Advanced Camera for Surveys aboard the {\it
Hubble Space Telescope} through the F606W and F814W filters, which brackets the
rest-frame 4000 \AA break at the cluster redshifts of z\sim 0.9. We fit the
zeropoint and slope of the red cluster sequence, and model the scatter about
this relation to estimate average galaxy ages and formation redshifts. We find
intrinsic scatters of 0.038-0.053 mag in ($V_{606}-I_{814}$) for the E and E+S0
populations, corresponding to average ages of 3.5-3.7 Gyr and formation
redshifts z_{f}=2.4-2.6. We find at least one significant difference between
the Cl1604+4304 and Cl1604+4321 early-type CMRs. Cl1604+4321, the less X-ray
luminous and massive of the two, lacks bright L^* ellipticals. We combine the
galaxy samples to fit a composite CMR down to 0.15L^*, and find that the slope
of the combined cluster CMR is significantly steeper than for RX J0152.7-1357
but consistent with MS 1054-03, both at similar redshift. The slope of the
Cl1604 CMR at the bright end (L > 0.5L^*) is flatter and consistent with the
CMR slopes found for other high redshift clusters. We find evidence for
increasing scatter with increasing magnitude along the early-type CMR,
consistent with a 'downsizing' scenario, indicating younger mean ages with
decreasing galaxy mass.

Astrophysics