  We present a general description of the interaction between multi-color laser
pulses and atoms and molecules, focusing on the experimentally relevant example
of infrared (IR) pulses overlapped with attosecond pulse trains (APTs). This
formulation reveals explicitly and analytically the role of the delay between
the IR pulse and APT as a coherent control parameter. Our formulation also
shows the nearly equivalent roles of the delay and the carrier-envelope phase
in controlling the interference between different multiphoton pathways. We
illustrate these points by investigating the single ionization of He and
introduce dressed adiabatic hyperspherical potentials to aid the discussion. We
confirm the predictions with a full-dimensional, two-electron solution of the
time-dependent Schr\"odinger equation.
