re: Only valuable plant color I can think of is indigo, first available in 1745.
do you mean commercially available in the america's or in europe? the people of ghana (ashanti) were early (if not the first) people to have extracted indigo blue dye. they have many myths about how it was first found, basicly all involve a woman who begs the gods for cloth the color of the sky, and promises she will give anything.
she is lure into the forest, directed to collect some leaves to make a bed for her son, to lie him on it, and to build a small fire to protect him.
she lies him down, and then is taken all about the forest.. she is gone for hours, and is worried about her son..
finaly she returns, and finds her son is burning.. the blanket she laid him on, caught fire, she runs about, looking for some water, and throws it on him, but its to late. Everything is burnt, only a small scrap of cloth remains.. it smells, and it is warm, still damp from urine and the water she clutches it, an cries.. and as her salt tear touch the cloth, it turn the color of the sky.. and so the gods tell her, thank you for the gift of your first born son... -- now you know the secret.. you must take this plants, crush them, and cook them in urine, heat them, wash them, and then add salt water..
heating (or cooking) urine will give you a mild ammonia, a common solvent for plant dyes, and salt is a common mordant. the leaves, of course are indigo plants.
indigo became a cash crop in US, but i think it was introduced to the western world as europians went to west africa in search of slaves.
There is a name for one shade of purple that was made up.. since it was a shade never found in nature. (i thought it was mauve, but i checked and its not it.) it was a 'brand name' for a color extract from coal tar dyes. anyone remember?
