Years ago. I went to a local quilt show, where one of the local guilds had a tea stain challenge. It was very interesting..some were lighter, some were very dark(achieved by strength of tea and how long you soak). There is also a method where you soak the muslin, then sprinkle salt on it to give it a mottled look.From what I remember the ladies telling me at the time, it is "semi-permanant"...meaning, once it is rinsed, then dried, it will keep most of it's color, but just like some other dyed fabrics, will lose some color over time.