Ever dye wool tartan

[Dollander]

How long do you think I should let it soak in the coffee/tea? A few hours? days? Weeks? Also would a diluted brown dye also work?

I was about to suggest that, using a diluted dye. I dyed a British army dress jacket black, and the instructions on Rit Studio website (from someone who dyed a suit purple for her son's Joker costume ) advised using two bottles of dye versus only one for cotton, rayon, linen, etc. So maybe if you tried using, say, a third of a bottle of brown dye? Better to have it come out too weak and have to do it again than too dark.

Oh, I almost forgot...the instructions are here: https://www.ritstudio.com/2015/10/26/joker-cosume/ Only I found that I didn't have to use the fabric refresher let alone putting it in the dryer (which I would certainly not advise with a belted plaid), as the only vinegar smell I noticed was when I re-pressed the bottom edges with a press cloth sprayed with vinegar water...and even that smell dissipated on its own in less than an hour.