  The determination of stellar metallicity and its gradient in external
galaxies is a difficult task, but crucial for the understanding of galaxy
formation and evolution. The color of the Red Giant Branch (RGB) can be used to
determine metallicities of stellar populations that have only shallow
photometry. We will quantify the relation between metallicity and color in the
widely used HST ACS filters F606W and F814W. We use a sample of globular
clusters from the ACS Globular Cluster Survey and measure their RGB color at
given absolute magnitudes to derive the color-metallicity relation. We
especially investigate the scatter and the uncertainties in this relation and
show its limitations. There is a clear relation between metallicity and RGB
color. A comparison with isochrones shows reasonably good agreement with BaSTI
models, a small offset to Dartmouth models, and a larger offset to Padua
models. Even for the best globular cluster data available, the metallicity of a
simple stellar population can be determined from the RGB alone only with an
accuracy of 0.3dex for [M/H]< -1, and 0.15dex for [M/H]> -1. For mixed
populations, as they are observed in external galaxies, the uncertainties will
be even larger due to uncertainties in extinction, age, etc. Therefore caution
is necessary when interpreting photometric metallicities.
