You were right. I completed the DOff assignment "Obtain Experimental Harghpeng Torpedo Launcher", and on a standard success I received a Mk XI Purple HP Torp. I also checked the Crafting window, and it's showing the Mk XI as purple, too.

Strange. Damage is the same on both the purple and the blue I've got mounted. So they just changed the color, but nothing else...

Too bad.