Yes, I thought about the fertilizer too.
Amino acids would also be present in natural ferts, I think.
The outer wrappers might be in contact with amino acids that way... Interesting that the experimental conditions were heat, moisture and acidity - which N ferts could cause as well. But this is surprising to me in regards the blue colors, because in dyes made from other natural material, blue color is the most difficult to get and requires really alkaline conditions in all cases I know of (including indigo, dye mushrooms etc.).
Sulfur dyes though were one of the great successes of chemical industry. They are quite light fast and wash fast, easily made from chemicals and iirc this is why they took the place of natural dyes as much cheaper.
I know onion skins are commonly used for natural dye, and produce brown or black colors. But I never heard of anyone using garlic juice to dye cloth blue or purple. I guess in former days, no one had access to anything like pure amino acids. Very tempted to do a few experiments.
