  We report new HST/WFPC2 photometry for 10 globular clusters (GC) in M31
observed in F5555W(V) and F814W(I). Additionally we have reanalyzed HST
archival data of comparable quality for 2 more GCs. Extraordinary care is taken
to account for the effects of blended stellar images and required field
subtraction. We thus reach 1 mag fainter than the horizontal branch (HB) even
in unfavorable cases. We present the color-magnitude diagrams (CMDs) and
discuss their main features also in comparison with the properties of the
Galactic GCs. This analysis is augmented with CMDs previously obtained and
discussed by Fusi Pecci et al. (1996) on 8 other M31 clusters. We report the
following significant results: 1. The locus of the red giant branches give
reliable metallicity determinations which compare generally very well with
ground-based integrated spectroscopic and photometric measures, as well as
giving good reddening estimates. 2. The HB morphologies show the same behavior
with metallicity as the Galactic GCs, with indications that the 2nd-parameter
effect can be present in some GCs of our sample. However, at [Fe/H] ~ -1.7 we
observe a number of GCs with red HB morphology such that the HB type versus
[Fe/H] relation is offset from the MW and resembles that of the Fornax dwarf
spheroidal galaxy. One explanation for the offset is that they are younger than
their MW counterparts by 1-2 Gyr. 3. The Mv(HB)-[Fe/H] relationship has been
determined and the slope (~0.20) is very similar to the values derived from RR
Lyrae stars in the MW and the LMC. The zero-point of this relation based on the
assumed distance modulus (m-M)o(M31)=24.47+/-0.03 is consistent with
(m-M)o(LMC)=18.55.
