I had a cotton fabric in a very sharp yellow tone that I wanted to make a bit duller. I also wanted to dye a piece of this white fabric more beige.
I put them together into a bowl of strong tea. Stirred, waited, stirred, lifted up, rinsed.
The result was horrible. The white fabric you see here: Brown spots.
What about the yellow fabric? It became yellow-brownish with the same brown spots. Kind of like a mix of pee and poo. I threw it away.
Conclusion: You never know with tea. But it might be worth trying. But not on your favourite piece of fabric.