  The observed characteristics of globular cluster (GC) systems, such as
metallicity distributions, are commonly used to place constraints on galaxy
formation models. However, obtaining reliable metallicity values is
particularly difficult because of our limited means to obtain high quality
spectroscopy of extragalactic GCs. Often, `color--metallicity relations' are
invoked to convert easier-to-obtain photometric measurements into
metallicities, but there is no consensus on what form these relations should
take. In this paper we make use of multiple photometric datasets and iron
metallicity values derived from applying full-spectrum stellar population
synthesis models to deep Keck/LRIS spectra of 177 GCs centrally located around
M87 to obtain a new color--metallicity relation. Our new relation differs
substantially from previous relations in the blue, and we present evidence that
the M87 relation differs from that of the Milky Way GCs, suggesting
environmental dependence of GC properties. We use our color--metallicity
relation to derive a new GC metallicity-host galaxy luminosity relation for red
and blue GCs and find a shallower relation for the blue GCs than what previous
work has found and that the metal-poor GCs are more enriched than what was
previously found. This could indicate that the progenitor satellite galaxies
that now make up the stellar halos of early-type galaxies are more massive and
formed later than previously thought, or that the properties of metal-poor GCs
are less dependent on their present-day host, indicating a common origin.
