Sansa's hairnet from Dontos (turned into a necklace on the show) had deep purple amethysts (they're pale blue stones on the show), one of which (the one Olenna Tyrell grabs from the necklace on the show) contained the poison that she puts in the also dark red/purple wine that kills Joffrey, with the additional dark humor reference to the fact his face turns dark red (in the book) and thus the fan-name of "purple wedding".