For the effect the phase difference is the thing which matters. However, the wave reflected from the back and the wave reflected from the front run through diff

For the effect the phase difference is the thing which matters. However, the wave reflected from the back and the wave reflected from the front run through different path length (twice the thickness). For the effect to work the waves need to be in phase along that distance which is a few µm for sunlight (look up coherence length). The different wavalengths make only up for color. Some wavelength will have destructive interference and go missing. As the eye is quite sensitive to that you will see a colored surface, whichs color changes with view angle (as different wavelengths will cancel for different angles).