Based on the frequency-domain theory in nonperturbative quantum electrodynamics, we investigate the above-threshold ionization of an atom exposed to two-color laser fields of different frequencies. As both the frequencies of two laser fields are lower than the ionization potential of the atom, the above-threshold ionization spectrum comes from quantum interference among many ionization channels, and the two laser fields play same roles in the above-threshold ionization process. As the frequency of one of the two laser fields increases, the above-threshold ionization spectrum shows a multiplateau structure gradually, which is from the contribution of atoms absorbing different photons of high-frequency laser field. As the frequency of one of the two laser fields is much higher than the atomic ionization potential, the two laser fields play different roles in the above-threshold ionization process, where the high-frequency laser field may determine the ionization probability and the low-frequency laser field may determine the width of each plateau that can be predicted by the energy conservation relationship.