  In view of the important contributions of red giants to the integrated light
in evolutionary synthesis calculations of old stellar populations in clusters
and galaxies, we provide a comparison of theoretical isochrones computed from
the Padova, Geneva, and Yale evolutionary tracks, respectively. Using the most
recent version of the color-calibrated library of theoretical stellar spectra
by Lejeune et al. (1999), the isochrones are converted in a uniform manner to
the observational color-magnitude diagrams, (M(T1), C-T1) and (M(I), V-I), and
compared with observed giant branches of typical globular clusters spanning a
large range of metallicities. We find that the three different isochrones of
the red giant branch provide significantly different slopes and curvatures,
where in general, the differences tend to be larger for metallicities
decreasing below [M/H] ~ -1. Throughout the full metallicity range (~
-2<[M/H]<0), the Yale isochrones are the only ones which show very good matches
of both the observed giant branches and the associated metallicity scale
derived from the color of the RGB, while the Padova isochrones exhibit
significant discrepancies for low-metallicities. The effects of these
differences on the integrated model colors of single stellar populations, in
particular for age and metallicity determinations, are briefly addressed.
