  The colors of globular clusters in most of large elliptical galaxies are
bimodal. This is generally taken as evidence for the presence of two cluster
subpopulations that have different geneses. However, here we find that, because
of the non-linear nature of the metallicity-to-color transformation, a coeval
group of old clusters with a unimodal metallicity spread can exhibit color
bimodality. The models of cluster colors indicate that the horizontal-branch
stars are the main drivers behind the empirical non-linearity. We show that the
scenario gives simple and cohesive explanations for all the key observations,
and could simplify theories of elliptical galaxy formation.
