I think that this song is supposed to be sung from from the soldier who is fighting in WWI. He seems to be singing to his love. He misses her and wants to her to fight with him in the trenches. I think that purple goes with red because red would represent both love and bloodshed, purple might represent purple like a purple heart, awarded to those who have died fighting. He dies in battle, and only the purple heart is brought back to her. "Say You'll Never Leave Us" might refer to him protecting and watching her even after he dies.
I think that this song is supposed to be sung from from the soldier who is fighting in WWI. He seems to be singing to his love. He misses her and wants to her to fight with him in the trenches. I think that purple goes with red because red would represent both love and bloodshed, purple might represent purple like a purple heart, awarded to those who have died fighting. He dies in battle, and only the purple heart is brought back to her. "Say You'll Never Leave Us" might refer to him protecting and watching her even after he dies.