Amazing info in this article. This lady has been researching pokeberries as dye for 18 years. And she has finally got it figured out.
No loss in color in Ancient fabric died w pokeberries.
Pokeberry colors range from bright fushia to magenta to raspberry, and will last for several years by following the below recipe. Three factors seem to greatly effect the color and color retention: the concentration of dyestuff to fiber, and the degree of acidity of the mordant and dye baths, and the length of time fibers remain in each step of the process.