Ok, found an old sleeveless t-shirt (some call it a w*** beater shirt). Cut up small sections, brewed some Twining's Black Tea (Earl Grey & Irish Breakfast) and brew a cup of black coffee from my Keurig K-Cup machine. Let the tea bags steep for around 10 minutes, wetted the pieces and let them soak in each cup for around five minutes. Took them out and gave them all a good rinse and laid them on a paper towel for a bit.
The coffee one came out a nice dark, but when I rinsed it all washed away and was rather light in tone. The teas kept their tones after a rinse, so I'll go with the tea method.
The Earl Grey was a little too light for my liking, but the Irish has a very nice darkened tone to it, so I'll be using that one. Glad I did those tests beforehand.
The picture makes them look darker than they are. Just my limited photo skills with my phones camera.
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