Yup, it is from EQUI_GroupWindow.xml. It's a modified version, of course. It combines the player window into it, to some extent, and also has xp gauges, but all else should be relatively standard.
I'm stumped. The default UI shows group leader in green just fine, as do some other UIs. Same for yellow mercenaries.
Why this window does not is a mystery to me.
EDIT: Just thought of, but I think that I mentioned this in the original post. If I am group leader, this group window shows *my* name in green just fine. The problem is that if anyone else is group leader, I can't tell who it is without asking.