Brown :-P

I've certainly been told that my face (as opposed to just my eyes) lights up when I see whoever it is I'm going out with; most memorably, in rather sour tones by someone else who wished it was still them, but nevertheless. From my own observation, people's faces look very different when their eyes are wide; it can take years off someone and totally change what you'd read of their character from their expression. I think maybe the description is a shorthand for that kind of flash of change.