Ice crystals on a car windscreen illuminated by a street lamp. Pictured in Northern Russia by Boris Borisovich (journal).
Image ©Boris Borisovich, shown with permission
Ice has a rather open hexagonal structure. We can thank that for our existence for otherwise ice would not float on water and Earth's climate would be quite different.
An ice crystal's properties depend on direction. It is anisotropic. As a result light refracts differently depending its direction through the crystal. Unpolarised light splits into two polarised components travelling in slightly different directions.
Waves of the differently refracted polarised components overlap and interfere. The effect is wavelength dependent and so we see birefringent colours.