We investigate the color-magnitude (CM) relation of galaxies in the distant X-ray-selected cluster RDCS 1252.9-2927 at z = 1.24 using images obtained with the Advanced Camera for Surveys 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 (i775-Z850) of 0.029 ± 0.007 mag based on 52 galaxies or 0.024 ± 0.008 mag for ∼30 elliptical galaxies. Simulations using the latest stellar population models indicate an age scatter for the elliptical galaxies of about 34%, with a mean age TL ≳ 2.6 Gyr (corresponding to ZL ≳ 2.7), and the last star formation occurring at Zend ≳ 1.5. 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 of evolution out to z ≈ 1.2. Thus, elliptical galaxies were already well established in X-ray-luminous clusters when the universe was a third of its present age.