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The color-magnitude relation has been determined for the RDCS J0910+5422cluster of galaxies at redshift z = 1.106. Cluster members were selected fromHST ACS images, combined with ground--based near--IR imaging and opticalspectroscopy. The observed early--type color--magnitude relation (CMR) in(i_775 -z_850) versus z_850 shows intrinsic scatters in color of 0.042 +/-0.010 mag and 0.044 +/- 0.020 mag for ellipticals and S0s, respectively. Fromthe scatter about the CMR, a mean luminosity--weighted age t > 3.3 Gyr (z > 3)is derived for the elliptical galaxies.  Strikingly, the S0 galaxies in RDCS J0910+5422 are systematically bluer in(i_775 - z_850) by 0.07 +/- 0.02 mag, with respect to the ellipticals. Theellipticity distribution as a function of color indicates that the face-on S0sin this particular cluster have likely been classified as elliptical. Thus, ifanything, the offset in color between the elliptical and S0 populations may beeven more significant.  The color offset between S0 and E corresponds to an age difference of ~1 Gyr,for a single-burst solar metallicity model. A solar metallicity model with anexponential decay in star formation will reproduce the offset for an age of 3.5Gyr, i.e. the S0s have evolved gradually from star forming progenitors.  The early--type population in this cluster appears to be still forming. Theblue early-type disk galaxies in RDCS J0910+5422 likely represent the directprogenitors of the more evolved S0s that follow the same red sequence asellipticals in other clusters.  Thirteen red galaxy pairs are observed and the galaxies associated in pairsconstitute ~40% of the CMR galaxies in this cluster.Comment: ApJ, in pres

Evolution of the Color‐Magnitude Relation in High‐Redshift Clusters: Blue Early‐Type Galaxies and Red Pairs in RDCS J0910+5422