I've not done any new dyeing, but here's what I did do...
I finished the 1864 shawl in brown and ballet pink corriedale yarn. The pink was the 4th batch of yarn to go into my pokeberry dyebath.
I theorized that the yarn that goes in first absorbs the majority of the tannins, leaving mostly just the pink dyes for subsequent batches of yarn. This could in fact be true.
For those who are new to this and don't know what I'm talking about, the problem with pokeberries is that they are believed to not produce a dye which is either lightfast or washfast. Lack of washfastness isn't a surprise really because apparently, no berry-based dye is washfast except cranberry. But the lightfastness was a problem because the yarn would brown and fade as the tannins were activated by sun light.
After finishing the shawl, I had a lot of yarn left over. So I took a three inch piece and taped it to one of our south-facing windows. It's been there all day long for four days now and has not browned or even faded. I just took it down a few moments ago and the color is not any different than the yarn in the shawl. So I can conclude that the majority of the tannins probably were absorbed by earlier dyebaths.
I have taped the yarn back up to the window and will continue checking it to see how long it does take for the color to fade.
As I use the other batches of yarn, I'll test each one in turn, provided that there is enough yarn left over from the project.
I would share pictures but my camera phone is being finicky and won't let me do that today.