  We have investigated the optical and near-infrared colors of K'-selected
galaxies in clusters at z ~ 1.2 near to the radio galaxy 3C 324 using images
obtained with the Subaru telescope and archival HST data. The distribution of
colors of the galaxies in the cluster region is found to be fairly broad, and
it may imply significant scatter in their star-formation histories, although
the effect of contamination of field galaxies is uncertain. The red sequence of
galaxies whose R-K colors are consistent with passive evolution models for old
galaxies is found to be truncated at K' ~ 20 mag, and there are few fainter
galaxies with similar red colors in the cluster region. We find that the
bulge-dominated galaxies selected by quantitative morphological classification
form a broad sequence in the color-magnitude diagram, whose slope is much
steeper than that expected from metallicity variations within a passively
evolving coeval galaxy population. We argue that the observed color-magnitude
sequence can be explained by metallicity and age variations, and the fainter
galaxies with K' > 20 mag may be 1-2 Gyr younger than the brighter galaxies.
Some spatial segregation of the color and K'-band luminosity is seen in the sky
distribution; the redder and the brighter objects tend to be located near 3C
324.
