I have a folding bathroom door with two ugly frosted glass panels. My other half would prefer them opaque.
I wondering what quick wins I can do here. Will some sort of glass paint work?
Removing paint from glass is pretty much always easy. It's hard to scrape it off if the surface is rough and frosted but then again, glass won't be adversely affected by paint removers. If you were to go with film, most of the adhesives that hold them on are pretty serious - always test first.
For this, I would use lots of thin layers of spray paint. This would make the surface appear smoother since I'm sure there's some texture with the frosting, and the spray paint will stick to the low spots and even up. Make sure to mask it off well!
I think paint would be a bad idea since removing it would involve a lot of labor. You almost want something like what would be used on a car wrap or window tints. I bet if you called a local print shop they could produce something in any color you'd like that could be applied to the glass and be easily removed (similar to a company logo that is applied to a window, but full size).