Best way to preserve pokeberry dye?

Just wondering what people recommend as the best way to fix pokeberry dye on clothes.  I'm in love with the brilliant magenta stain it gives and my kids and I had fun making a tie-dye t-shirt with some of the berries last summer.  I know most natural dyes fade or go muddy so I still have it in the laundry room until I know if there's a best way to preserve the gorgeous color.  :)  

I was thinking that we could just take it as a fact of life that it will fade/change and just keep using new stuff to make a sort of ever-changing natural stain shirt, but I'd also love be able to use pokeberries to give new beauty to stained dishcloths and stuff like that, even if it's a paler version in the end.  Any experience?

As a kid, I loved to play with pokeberries.  Every summer I just could not wait until the plant by our backdoor got the berries.

I would try dying something like a scrap piece of material for an experiment. Then after the scrap has dried soak it in one gallon of water with 1/4 cup table salt and one cup vinegar.  

I had heard or mordant though I did not know what it was:

https://www.motherearthnews.com/diy/plant-based-mordants-ze0z1312zbla/

Can you dry/dehydrate the berries and grind them into a powder?