And what are my impressions so far after these experiments? Nettles - disappointment. The nettles I used were already old so that might have been the problem. The color was just not strong enough :(. Red onion skins - works good both on un-mordanted or mordanted wool, and you can get a nice color even with not very much plant material. As for the coffee experiment - hm... I am not very impressed with the color I got. But of course, that is just a matter of taste.