We carried out a wavelength-dependent homodyne and heterodyne transient grating (TG) studies on a resonant system, rhodamine 640 in dimethylsulfoxide, with a simplified diffractive optics method. The real and imaginary components of the third-order molecular polarization at different excitation wavelengths have a similar longtime decay (>200 fs), while the amplitude of the real component is strongly wavelength dependent. The nonresonant solvent and the resonant solute contributions to the third-order polarization are found to have a similar magnitude in a dilute solution. The interference of the wavelength-dependent resonant solute contributions with the nonresonant solvent contribution is found to be crucial to properly understand the wavelength dependence of the initial dynamics in homodyne TG and other four-wave mixing techniques. Such an interference effect is further confirmed by concentration-dependent homodyne TG measurements.