  We numerically investigate how fundamental properties of the spectral energy
distribution (SED) of a major gas-rich galaxy merger with dusty starburst are
determined by the initial orbital configuration of the merger. We found that an
infrared luminous galaxy with dusty starburst formed by a nearly
retrograde-retrograde merger suffers the most remarkable dust extinction of
stellar light and consequently shows very red colors. Considering that a
retrograde-retrograde merger does not produce strong and long tidal tails, this
result suggests that a luminous infrared galaxy without clear signs of
interaction and merging shows very large internal dust extinction and redder
colors.These numerical results furthermore imply that the morphology of a
luminous infrared galaxy can correlate with the degree of internal dust
extinction and thus with the shape of the SED, principally because the
morphology of a merger also depends strongly on the initial orbital
configuration of the merger.
