Polarization and Interference Colors
When light is passed through a polarizer to produced linearly polarized light and that light is then passed through a piece of birefringent material, the light is broken up into two components. Since the index of refraction of one of them is larger, that component will lag in phase. Then if the light is passed through a crossed polarizer, only that part of each of the components which is in the transmission plane will emerge. That means you have two coplanar components with a phase difference. If different colors have different indices, then for a given thickness of birefringent material, some colors will undergo destructive interference and some constructive, giving an interference pattern of varying colors reminiscent of the interference colors of a thin film.
Optics, Sec 8.9