Maybe the paper bag breathes more? I've speed-ripened avocados in those plastic produce bags they have in the produce section with twist ties and it seems to work okay

It has to be paper to let humidity pass out. Fruit will rot if kept in a plastic bag for too long because of the humidity build-up.

It doesn't have to be brown, it's just that the classic paper bag you buy at the store is always brown. You know, the lunch bags for kids (or adults). I've never seen them sold to consumers in any other color.

You could reuse a white bleached paper bag that you sometimes get with food delivery, but they always seem to acquire a grease stain or other liquid along the way... But the food delivery bags are often waxed, and that won't let humidity pass either.

They mean the bags made of uncoated kraft paper, which lets air in very well. "Brown [paper] bag" is essentially the name of this type of bag, which is often used for school lunches (small bags) or groceries (bigger). Here's an example:

https://www.canadiantire.ca/en/pdp/goodtimes-paper-lunch-bag...

They come in other colors--we had bleached white ones when I was a kid--and those would be fine too. However, it risks confusion with other paper bags, like the ones used in bakeries. These often have some kind of liner (polyethylene, plastic, foil) or wax coating that would impede airflow.

The department store paper bags, even if they are pure paper and colored brown, are often multiple layers that would block airflow too.

Just oxygen-permeable. I also keep my cheese in an air-permeable bag in the fridge - if there's no bag, it dries too fast, if the bag is not air-permable the cheese gets sweaty and gross.