Ellipsometric studies are performed for opal-like photonic crystals made of polystyrene microspheres. For the first time, we discuss spectral behavior of the phase shift delta between the p- and s-polarization components of complex-valued reflection coefficients for the case when the multiple Bragg diffraction of light inside a photonic crystal takes place. The measured spectra of delta demonstrate a strong frequency dependence which is analyzed in the framework of the dynamical diffraction theory. Diffusely scattered light from the bulk and surface imperfections of photonic crystal structure is shown to influence the resonant spectra of delta.