I have used salt vinegar or soda to set natural dyes when I have been dying my own fabrics. These are dyes made from natural things like Gum tree bark, or leaves, Walnut skins, onion skins etc flower heads etc. These dyes work best on natural fibers, like cotton silk wool, it does work, and heat is used to set the colour also. Many people are disappointed with the results only because they are used to seeing synthetic dyes which have intense colours. Organic naturals dyes are very soft and subtle and because of this one can be forgiven for thinking it hasn't taken.
Gal