Turns out it was hiding in plain sight by the side of a Portuguese road.
During hot, dry summers in southern Portugal, the key ingredient for medieval manuscripts grows by the roadside. It is called folium, or turnsole, and it’s derived from the fruit of Chrozophora tinctoria, a small plant that grows in the region. For centuries, folium was responsible for coloring everything from Bible scenes to, later, the rind of a popular Dutch cheese. AtlasObscura.com