I had a new knit shirt in a sort of cold-oatmeal heather color that I wanted to make a deeper and warmer color. I bought some cheap tea, which costs about $2 for 100 bags, and brewed 30 or 40 bags in a stock pot. I took the bags out before putting in the shirt, and stirred it for a few minutes, until the color looked about right. Then I took it out and hung it up outside on a hanger, trying to straighten the fabric as much as possible--for more even drying and less streaking. After drying, I rinsed it and put it through the dryer. Since then, it has gone through the regular laundry without much fading. Things that I tea-dyed a couple of years ago have held the color all right.
With the same batch, I also over-dyed a colored sleeveless shirt, just slightly. It was much slower to pick up tea coloring than the first shirt was.
I've found that newer cotton fabrics work best. Next time I am going to do the rinsing and machine drying right away, because the first shirt that I dyed this time still came out a little streaky under the arms.