CH free radicals in an oxyacetylene flame at atmospheric pressure were detected by means of the two-color resonant four-wave mixing technique. Ground-state grating schemes with omega(1)=omega(2) (pump) and omega(3) (probe)=omega(4) (signal) were used; omega(1)(=omega(2)) and omega(3) are in resonance with distinct rovibronic transitions of B (2) Sigma(-) - X (2) Pi and A (2) Delta - X (2) Pi Of CH, respectively. Varying the relative polarization of the pump and probe beams significantly affects the ratio of signal to noise. Two schemes with cross polarization pairs (omega(1) perpendicular to omega(2)) and (omega(3) perpendicular to omega(4)) produced results best suited to detect trace species in the luminous, dense medium; the relative J dependence observed for these schemes agrees with theoretical predictions. The double-resonance feature of this technique, advantageous to identify congested lines, is also demonstrated. When an A - X transition line is employed as a probe, previously unobserved predissociative levels of B (2) Sigma(-) are detected; the R(12) line of B<--X (1,0) transion has a halfwidth greater than 10 cm(-1), corresponding to a lifetime of 0.5 ps.