U--R colors of elliptical and SO galaxies have been synthesized by combining the stellar populations of galactic globular clusters in accordance with a simple mass-loss chemical evolutionary model. We can account in a satisfactory way for the range, shape, and dispersion of the color-magnitude relation observed by Sandage and Visvanathan. The model independently predicts the mass and metal abundance of the intracluster medium, which agrees with recent model dependent interpretatins of the X-ray observations of clusters of galaxies.