Perfect Memento (in the wiki), though he exaggerated a bit. Youmu body temperature is low compared to a normal human but she's not cold and Myon's "body isn't particularly cold to the touch" meaning that it's not cold but it's not warm either....cool would be the best description, which is why she makes for the perfect comforter in summer.
that just means as a half ghost she's only half as cold/warm as full ghosts/humans. yuyuko is still a full ghost herself who should be colder than her.
Yeah, actually that's a pretty good way to put it! But also remember, everyone calls her a half ghost (guilty of this too), but she's actually half phantom which for the most part is the same but different at the same time (weird). Unfortunately, this difference isn't clearly explained except for the fact that phantoms don't necessarily come from the dead and can simply appear. (ie: Mima and the Prismriver sisters)