Purple Grail is a cup obtained in the purple tower of the Castle of the Prism. It is associated with eternal love: “For this we will toast, with our eternal love”.
After the towers it is used to give drink to Atropal Matt, the baby (“Give me the cup, and I run to them”). The baby drinks for the first time.
The grail connects the test of love (purple) with the test of paternity (orange).