  Morphological aspects of the $V, \bv$ diagram of metal-rich globular clusters
are analyzed, on the basis of stellar evolution models for horizontal-branch
(HB) and red giant branch stars. These have been incorporated into detailed
synthetic HB models, the influence of differential reddening upon which is also
discussed. The synthetic HB models are found to be very clumpy in the
colour-magnitude diagram, though more extended structures occasionally result
due to the effect of evolution away from the zero-age HB. In particular,
"sloped" clumps may be naturally expected for sufficiently high helium
abundances. Differential reddening is found to have a smaller influence than
evolution itself upon the morphology of the clump. The synthetic HB models are
also used to study the age difference between metal-rich clusters on the basis
of the "$\Delta V$" method. We emphasize the importance of knowing in advance
the relative abundances of helium and metals in order to estimate this age
difference quantitatively. We present chemical evolution models that serve to
illustrate the effect, and show that, for two clusters of identical [Fe/H] and
$\Delta V$, the one enriched predominantly by ejecta from supernovae type II
explosions will appear younger than the other which also suffered enrichment
from supernovae type Ia ejecta. The implied locations of the red giant branch
feature known as the "bump" are also discussed on the basis of recent
observational and theoretical results. Our corresponding predictions should be
checked against accurate observational data obtained with the Hubble Space
Telescope.
