  We have tested the reliability of the red giant branch (RGB) as a metallicity
indicator accounting for observational errors as well as the complexity of star
formation histories (SFHs) and chemical evolution histories observed in various
stellar systems. We generate model color-magnitude diagrams (CMDs) produced
with a variety of evolutionary histories and compare the resultant metallicity
estimates from the colors and magnitudes of RGB stars to the true input
metallicities. We include realistic models for photometric errors and
completeness in our synthetic CMDs. As expected, for simple simple stellar
populations dominated by old stars, the RGB provides a very accurate estimate
of the modular metallicity value for a population. An error in the age of a
system targeted for this type of study may produce metallicity errors of a few
tenths of a dex. The size of this metallicity error depends linearly on the age
error, and we find this dependence to be stronger with more precise photometry.
If the population has experienced any significant star formation within the
last $\sim$6 Gyr, the metallicity estimates, [M/H], derived from the RGB may be
in error by up to $\sim$0.5 dex. Perhaps the most important consideration for
this technique is an accurate, independent estimate of the average age for the
target stellar system, especially if it is probable that a significant fraction
of the population formed less than $\sim$6 Gyr ago.
