I keep wanting to spray mine silver and stick a plunger to its side
Yep, brown cardboard boxes (torn up into small pieces), or if you ever have Amazon packaging the rough brown paper that they use as padding is even better. Scrunch it into balls to allow air in.
Straw, woodchop, well broken branches also work (though wood breaks down slower) and a lot of people use scrunched newspaper though I’m always a bit weary because of ink… though apparently the use of problematic inks has long since passed, but for some reason I always avoid it.
1/3 of the stuff in your compost bin should be ‘brown’ materials, which aids the breakdown and stops it turning into smelly slurry. I also have a composting enzyme that you add a little to a watering can of water to wet the pile.
Grass is an odd one in that if you add the fresh clippings it is green, but if you wait for it to dry and turn brown it is brown.
I realise that I’ve just said that green is green and brown is brown, but you know what I mean