Mine never go brown and I’ll tell you why. You see my father has inherited this way of doing things from my great great grandfather in law, and this is how I do it. First thing when I buy them, I hang them in my dining room upside down from the chandelier, then I leave them ther for 2.6 hours and then bake them at 350 for ten minutes. Then when I put them in the lilav sleeve, I wrap it in a yellow towel, that I saturate with exactly 2 cups of water mixed with 1 tbs of vinigar and 1 shot of triple sec. And put it in a moldy lulav case that has my lulav from the year before (I keep it there year to year). Use green rubber bands to keep the towel on.
This works so well, I never buy more than 1 set of aravos a year. Also, no one sits near me during davening so I have plenty of space to do my naanuim (I hold according to the shita that says you need to walk 4 feet three times in each direction). So far I have not had any problems, so if anyone would like to try it, I would love to know what you think of it.