We demonstrate very high-contrast coherent population trapping[1] (CPT) resonances by using four-wave mixing in ${}^{87}\mathrm{Rb}$ atoms. In the experiment, we take advantage of the spectral overlap between $F=2\to F^{\prime }$ and $F=3\to F^{\prime }$ optical resonances on the $D1$ line of ${}^{87}\mathrm{Rb}$ and ${}^{85}\mathrm{Rb}$ atoms, respectively, to eliminate the DC-light background from the CPT resonance signal. We observe a CPT resonance with a contrast in the range of 90%, compared with a few percent achieved by alternative methods.