It depends on what kind of purple you want to get - I tested with madder root last week ( got more than 100 colors with different toning! almost like magic, from pink to dark red, even got some really beautiful purple! )

but you can also get different types of purple from logwood. 

Natalia

Stacker Saloon

saloon

The anon has already given a clue and I also think it has to do with dyeing clothes. I'm curious because last week I was looking at the history of the color purple and how it was associated with nobles, because it was expensive to make that color. I learned that it was extracted from a sea snail. cc\ @orthwyrm :)