  (Abridged.) We present F435W (B), F606W (Broad V), and F814W (Broad I)
coronagraphic images of the debris disk around Beta Pictoris obtained with
HST's Advanced Camera for Surveys. We confirm that the previously reported warp
in the inner disk is a distinct secondary disk inclined by ~5 deg from the main
disk. The main disk's northeast extension is linear from 80 to 250 AU, but the
southwest extension is distinctly bowed with an amplitude of ~1 AU over the
same region. Both extensions of the secondary disk appear linear, but not
collinear, from 80 to 150 AU. Within ~120 AU of the star, the main disk is ~50%
thinner than previously reported. The surface-brightness profiles along the
spine of the main disk are fitted with four distinct radial power laws between
40 and 250 AU, while those of the secondary disk between 80 and 150 AU are
fitted with single power laws. These discrepancies suggest that the two disks
have different grain compositions or size distributions. The F606W/F435W and
F814W/F435W flux ratios of the composite disk are nonuniform and asymmetric
about both projected axes of the disk. Within ~120 AU, the m_F435W-m_F606W and
m_F435W-m_F814W colors along the spine of the main disk are ~10% and ~20%
redder, respectively, than those of Beta Pic. These colors increasingly redden
beyond ~120 AU, becoming 25% and 40% redder, respectively, than the star at 250
AU. We compare the observed red colors within ~120 AU with the simulated colors
of non-icy grains having a radial number density ~r^-3 and different
compositions, porosities, and minimum grain sizes. The observed colors are
consistent with those of compact or moderately porous grains of astronomical
silicate and/or graphite with sizes >0.15-0.20 um, but the colors are
inconsistent with the blue colors expected from grains with porosities >90%.
