I have studied and researched into natural dyes lightfastness and the dyes used in my Landscapes are as lightfast as I can make them . Natural dyes vary in lightfastness and the blues from Indigo )reds & oranges from Madder and Cochineal are the most lightfast and I use weld and fustic as my yellows as these have by far the best lightfastness.
I recomend my Pictures are hung out of direct sunlight and then I would expect that they willl outast any of us.
The inks are graded from 1-3 after I exposed painted samples to one months in south facing window during a very sunny September. ! represents no fading, 2 some loss of colour and 3 signinificant loss of colour. All the inks are individually rated and all the ones I sell are rated either 1 ( blue, black) or 2.
Dyes and natural mordants sold as grown and dried in North Wales come from either the Blaen Wern Dye garden or the Gwaenynog Garden, where no chemicals are used.