  Globular Cluster (GC) systems of most early-type galaxies show bimodal
optical color distributions, yet their Luminosity Functions (LFs) look like
simple Gaussians with turn-over magnitudes used for determination of distances
and the Hubble constant. Taking color distributions and LFs of Milky Way halo
GCs as a reference for the apparently universal old metal-poor GC populations
and assuming their intrinsic widths to be constant in time at the values of the
Milky Way halo GC populations, I study for which combinations of age and
metallicity a secondary GC population formed in some violent star formation
event in the history of a galaxy may or may not be detected in the observed GC
color distributions and LFs (U- through K-bands). Significant differences are
found among the diagnostic efficiencies in various wavelength regions with the
NIR predicted to be able to reveal the presence of GC subpopulations that may
perfectly hide within one inconspicuous optical color peak. If, and only if,
some broad age -- metallicity relation is assumed, as expected e.g. for
secondary GCs forming in spiral galaxy mergers from a pre-enriched ISM, models
indicate single-peak GC LFs even in GC systems with bimodal color
distributions. We point out the importance of having multi-band information
(including NIR) to independently constrain ages and metallicities of GC
subpopulations that will shed light on both cluster and galaxy formation
scenarios. Models presented here are being used for the interpretation of
multi-wavelength data from our ASTROVIRTEL project on GC systems in galaxies of
various types, luminosities and environments as well as on young star cluster
systems in interacting galaxies and mergers.
