As colour words in English, "violet" and "lilac" are definitely NOT interchangeable: the colours of the flowers are very different, as are the colours designated by the words. Is "lila" the GENERAL all-inclusive Swedish word for purple, both dark shades and light shades? My dictionary gives "purpur".
Yes, indeed :) But lila is also used as a color name (a pale violet color).
"Pourpre" is also well known for its religious connotation (cardinal clothes) but it is a lot darker than the "lila" color. In fact, I think that the translation here is a bit wrong "Klänning är lila" -> "The dress is purple", because the Swedish "lila" is actually the same as the French "lila" and not as dark as the English "purple", lilac might be more accurate (nitpicking though).