Uhh... does the teen girl in question like green?
If you can't just ask her (like if painting the room is a surprise), try to guage her favourite colour based on other stuff she has... her sheets, backpack, favourite clothes, etc.
i am a 14 year old girl, and my walls are painted light green, and i like it
mmy fav color is dark purple and i painted my wall that color and it is sweet it matches my
laptop cell phone etc.