Yes, phlegmatic has roots in that colorless, mucousy stuff called phlegm, but people who are phlegmatic aren't called that because they have lots of mucus. They are just a little dull in expressing feelings or showing emotion.
It may be their training more than their natural behavior, but those palace guards who wear the red coats and big hats and show absolutely no expression on their faces are phlegmatic. Attempts to make them laugh, smile, or twist their faces in irritation won’t work, because being phlegmatic is important to their role as stone-faced keepers of the palace. Phlegmatic people show less emotion on the outside — but who knows, they may be jumping up and down on the inside.