Most dictionaries try to hedge this by saying a dark grayish blue or purple, but that would be an indigo, yes? Yes. The word has been in use since at least the 14th century, adopted, either directly or through an Anglo-Norman-French connection, from Medieval Latin pursus or persus, which seems ultimately to be an alteration of Latin Persicus or Persa, Persian. Latin in turn got the word from Greek, which got it from the Persians themselves. (Yes, the Persians actually called themselves Persians, or at least Pārsa -- the Greeks adapted the vowels during the adoption, as one does.) Why the color was considered Persian, I cannot find -- anyone with a hint here?
(That in Finnish perse is a very rude word, I am ignoring.)