We address high-order harmonic generation with linearly polarized bichromat
ic fields, concentrating on a modulation in the harmonic yield as a functio
n of the relative phase between the two field components, and on an offset
phase shift of this modulation for neighboring cutoff harmonics. These effe
cts have been recently observed in experiments where the relative phase bet
ween the two driving fields was controlled. Using the three-step model and
the fully numerical solution of the time-dependent Schrodinger equation, we
discuss the phase-dependent modulation and show that the offset phase is i
nherent to a particular set of semiclassical trajectories for the returning
electron. These trajectories are identified using classical arguments and
isolated by means of the saddle-point method, which allows a detailed inves
tigation of their interference. Thus, by adding a secund driving field whos
e amplitude Lies within an adequate parameter range, one is able to single
out a set of trajectories according to its behavior with respect to the rel
ative phase. This effect is already present at the the single-atom-response
level.