  We develop theoretically and confirm both numerically and experimentally a
comprehensive analytical model which describes the propensity rules in the
emission of circularly polarized high harmonics by systems driven by two-color
counter-rotating fields, a fundamental and its second harmonic. We identify and
confirm the three propensity rules responsible for the contrast between the
3N+1 and 3N+2 harmonic lines in the HHG spectra of noble gas atoms. We
demonstrate how these rules depend on the laser parameters and how they can be
used in the experiment to shape the polarization properties of the emitted
attosecond pulses.
