It is only the front part, and it is due to being polarized glass, which only manifests when the car is wet. I think it is kind of pretty actually (note that I own a Model 3).

The Model 3 has gone through several iterations of glass, and some builds actually have nearly the entire roof (including the back glass) turn orange in the rain, but I believe new builds actually have no coloration at all since they changed the glass again.

It isn't polarized, it's just an IR filter.