( So you understand, the " Hulls" are the soft Green- turning to Black" outer "skin " of the nut.
The harder " SHELL" that protects the actual meat of the walnut does not contain the dye. The Shells are ground up into a fine power and sold to be used as a brass case cleaning medium for cartridge cases. Its also used as abrasive in removing paint, graffiti, etc. and even used to remove paint in the auto repair industry. I believe its beginning to replace "sand" for " sandblasting stone buildings, too.
YOu can boil the hulls to quickly get the dark walnut dye out of them, and reduce the amount of water in the mix to get a thick black liquid/paste to apply, OR, you can simply put the hulls in some container with water, put a lid on it to conserve the water, and let it sit for several months. Then strain the contents.
Whether you boil or just let the dye leach out of the hulls over time, you will want to strain the mixture, through cheese cloth and screen wire. You can store the mix in any old jar you have with a lid on it. The stuff lasts for years. It is acidic, so store it with that in mind.
The acidic nature of this dye is why you have been cautioned about using it with wool, and the fact that wool will shrink if heated is why its suggested that if you go ahead and dye your old hat, do it with the dye mix and the hat "COLD". You do need to use a Mordant- you can google that to find common choices-- to set the dye, however, or it will streak down over your hair and head when the hat next gets wet( like from sweat!)
Personally, I would not ruin an old hat like you describe, that is just beginning to show " Character". If you want a new-looking hat, buy a new hat! But, then, again, I am not the kind of guy who is going to be sitting around for Botox treatments to my age lines on my face, either. :shocked2: :surrender: :thumbsup: