A golden yellow natural dye that can also be used to make greens depending on what is added to it. Weld is also known as Reseda luteola or by the common name dyer's rocket. It actually can grown as a weed, though it has been cultivated commercially in the past.
Sasha Duerr, in her book Natural Palettes noted that "Weld was used intensively as a dye in Medieval Europe, particularly in France... The widespread commercial use of weld as a dye came to an end in the early twentieth century, when synthetic dyes came into fashion."
More information can be found here at Kremer Pigments
