  We investigate the color-magnitude (CM) relation of galaxies in the distant
X-ray selected cluster RDCS1252.9-2927 at z=1.24 using images obtained with the
Advanced Camera for Surveys (ACS) on the Hubble Space Telescope in the F775W
and F850LP bandpasses. We select galaxies based on morphological
classifications extending about 3.5 mag down the galaxy luminosity function,
augmented by spectroscopic membership information. At the core of the cluster
is an extensive early-type galaxy population surrounding a central pair of
galaxies that show signs of dynamical interaction. The early-type population
defines a tight sequence in the CM diagram, with an intrinsic scatter in
observed (i-z) of $0.029\pm0.007$ mag based on 52 galaxies, or $0.024\pm0.008$
mag for $\sim30$ ellipticals. Simulations using the latest stellar population
models indicate an age scatter for the ellipticals of about 34%, with a mean
age $\taul\gta2.6$ Gyr (corresponding to $z_L\gta2.7$), and the last star
formation occurring at $z_{end}\gta1.5$. Transforming to rest-frame $(U-B)$, we
conclude that the slope and scatter in the CM relation for morphologically
selected early-type galaxies show little or no evidence for evolution out to
$z\approx1.2$. Thus, elliptical galaxies were already well established in X-ray
luminous clusters when the universe was a third of its present age.
