A method of chromatic polarization imaging is presented for the online detection of colorless plastic contaminants from ginned cotton in an industrial setting. To understand the experimental results, we consider a realistic microscopic model, including the multiple scattering of anisotropic fibers and the light propagation in anisotropic slabs. A Monte Carlo code, based on the extended Jones matrix, is developed to simulate photon migration with polarization states, and phase information followed. Using simulations and experiments, we analyze the underlying mechanisms and evaluate the performance of this method with different layer thicknesses. Our approaches proposed in this Letter also have the potential to be applied in tissue imaging, remote sensing, and other scenarios.