  Two recent empirical developments in the study of extragalactic globular
cluster (GC) populations are the color-magnitude relation of the blue GCs (the
"blue tilt") and the nonlinearity of the dependence of optical GC colors on
metallicity. The color-magnitude relation, interpreted as a mass-metallicity
relation, is thought to be a consequence of self-enrichment. Nonlinear
color-metallicity relations have been shown to produce bimodal color
distributions from unimodal metallicity distributions. We simulate GC
populations including both a mass-metallicity scaling relation and nonlinear
color-metallicity relations motivated by theory and observations. Depending on
the assumed range of metallicities and the width of the GC luminosity function
(GCLF), we find that the simulated populations can have bimodal color
distributions with a "blue tilt" similar to observations, even though the
metallicity distribution appears unimodal. The models that produce these
features have the relatively high mean GC metallicities and nearly equal blue
and red peaks characteristic of giant elliptical galaxies. The blue tilt is
less apparent in the models with metallicities typical of dwarf ellipticals;
the narrower GCLF in these galaxies has an even bigger effect in reducing the
significance of their color-magnitude slopes. We critically examine the
evidence for nonlinearity versus bimodal metallicities as explanations for the
characteristic double-peaked color histograms of giant ellipticals and conclude
that the question remains open. We discuss the prospects for further
theoretical and observational progress in constraining the models presented
here and for uncovering the true metallicity distributions of extragalactic
globular cluster systems.
