  Hubble Space Telescope observations with Wide Field Camera 3/IR reveal that
galaxies at z~7 have very blue ultraviolet (UV) colors, consistent with these
systems being dominated by young stellar populations with moderate or little
attenuation by dust. We investigate UV and optical properties of the high-z
galaxies in the standard cold dark matter model using a high-resolution
adaptive mesh refinement cosmological hydrodynamic simulation. For this
purpose, we perform panchromatic three-dimensional dust radiative transfer
calculations on 198 galaxies of stellar mass 5x10^8-3x10^{10} Msun with three
parameters, the dust-to-metal ratio, the extinction curve, and the fraction of
directly escaped light from stars (\fesc). Our stellar mass function is found
to be in broad agreement with Gonzalez et al., independent of these parameters.
We find that our heavily dust-attenuated galaxies (A_V~1.8) can also reasonably
match modest UV-optical colors, blue UV slopes, as well as UV luminosity
functions, provided that a significant fraction (~10%) of light directly
escapes from them. The observed UV slope and scatter are better explained with
an Small Magellanic Cloud-type extinction curve, whereas Milky Way-type dust
predicts too blue UV colors due to the 2175A bump. We expect that upcoming
observations by ALMA will be able to test this heavily obscured model.
