  We combine new Wide Field Camera~3 IR Channel (WFC3/IR) F160W (H) imaging
data for NGC1399, the central galaxy in the Fornax cluster, with archival F475W
(g), F606W (V), F814W (I), and F850LP (z) optical data from the Advanced Camera
for Surveys (ACS). The purely optical g-I, V-I, and g-z colors of NGC1399's
rich globular cluster (GC) system exhibit clear bimodality, at least for
magnitudes $I_814 > 21.5$. The optical-IR I-H color distribution appears
unimodal, and this impression is confirmed by mixture modeling analysis. The
V-H colors show marginal evidence for bimodality, consistent with bimodality in
V-I and unimodality in I-H. If bimodality is imposed for I-H with a double
Gaussian model, the preferred blue/red split differs from that for optical
colors; these "differing bimodalities" mean that the optical and optical-IR
colors cannot both be linearly proportional to metallicity. Consistent with the
differing color distributions, the dependence of I-H on g-I for the matched GC
sample is significantly nonlinear, with an inflection point near the trough in
the g-I color distribution; the result is similar for the I-H dependence on g-z
colors taken from the ACS Fornax Cluster Survey. These g-z colors have been
calibrated empirically against metallicity; applying this calibration yields a
continuous, skewed, but single-peaked metallicity distribution. Taken together,
these results indicate that nonlinear color-metallicity relations play an
important role in shaping the observed bimodal distributions of optical colors
in extragalactic GC systems.
