  Hubble Space Telescope imaging for 26 giant early-type galaxies, all drawn
from the MAST archive, is used to carry out photometry of their surrounding
globular cluster (GC) systems. Most of these targets are Brightest Cluster
Galaxies (BCGs) and their distances range from 24 to 210 Mpc. The catalogs of
photometry, completed with DOLPHOT, are publicly available. The GC color
indices are converted to [Fe/H] through a combination of 12-Gyr SSP (Single
Stellar Population) models and direct spectroscopic calibration of the fiducial
color index (F475W-F850LP). All the resulting metallicity distribution
functions (MDFs) can be accurately matched by bimodal Gaussian functions. The
GC systems in all the galaxies also exhibit shallow metallicity gradients with
projected galactocentric distance that average $Z \sim R_{gc}^{-0.3}$. Several
parameters of the MDFs including the means, dispersions, and blue/red fractions
are summarized. Perhaps the most interesting new result is the trend of
blue/red GC fraction with galaxy mass, which connects with predictions from
recent simulations of GC formation within hierarchical assembly of large
galaxies. The observed trend reveals two major transition stages: for low-mass
galaxies, the metal-rich (red) GC fraction $f(red)$ increases steadily with
galaxy mass, until halo mass $M_h \simeq 3 \times 10^{12} M_{\odot}$. Above
this point, more than half the metal-poor (blue) GCs come from accreted
satellites and $f(red)$ starts declining. But above a still higher transition
point near $M_h \simeq 10^{14} M_{\odot}$, the data hint that $f(red)$ may
start to increase again because the metal-rich GCs also become dominated by
accreted systems.
