  We theoretically investigate the optical energy absorption of crystalline
silicon subject to dual-color femtosecond laser pulses, using the
time-dependent density functional theory (TDDFT). We employ the modified
Becke-Johnson (mBJ) exchange-correlation potential which reproduces the
experimental direct bandgap energy $E_g$. We consider situations where the one
color is in the ultraviolet (UV) range above $E_g$ and the other in the
infrared (IR) range below it. The energy deposition is examined as a function
of mixing ratio $\eta$ of the two colors with the total pulse energy conserved.
Energy transfer from the laser pulse to the electronic system in silicon is
dramatically enhanced by simultaneous dual-color irradiation and maximized at
$\eta\sim 0.5$. Increased is the number of generated carriers, not the absorbed
energy per carrier. The effect is more efficient for lower IR photon energy, or
equivalently, larger vector-potential amplitude. As the underlying mechanism is
identified the interplay between intraband electron motion in the {\it valence}
band (before excitation) driven by the IR component and resonant
valence-to-conduction interband excitation (carrier injection) induced by the
UV component. The former increases excitable electrons which pass through the
$k$ points of resonant transitions. The effect of different multiphoton
absorption paths or intraband motion of carriers generated in the conduction
band play a minor role.
