  We have constructed synthetic color-magnitude diagrams (CMDs) for omega Cen
and other massive globular clusters with apparently peculiar CMD morphology.
Our population models, which adopt the most up-to-date input physics and
parameters, show that the observed CMD of omega Cen can be reproduced by
adopting (1) multimodal metallicity distribution function as derived from the
observed color distribution of red-giant-branch (RGB) stars, and (2) an
internal age-metallicty relation among the populations therein. Similar results
were obtained for other massive globular clusters with bimodal
horizontal-branches (HBs). In particular, we found that the peculiar CMD
morphology (broad RGB, bimodal HB) and properties of RR Lyrae stars observed in
NGC 6388 and NGC 6441 can be reproduced by the composite of two distinct
populations with mild internal age-metallicity relations. This suggests that
these clusters, as well as omega Cen, may represent the relicts of disrupted
dwarf galaxies.
