Dd1 (15) has dove grey walls with a floral patterned paper covering the chimney breast (black with silver/grey vines and dark pink flowers). She has damask curtains in shades of grey to match and a small black "chandelier" with a grey carpet. She chose but with input from me (she asked my opinion). Started with the patterned wallpaper and then picked the rest to compliment it.
Give them each a task to design their own room - either with pen and paper or via free software online like ikea or thebrick.icovia.com/ so they can have input too - not only will it keep them out of your way for a while - you will see who likes what and what, if anything , is a good idea or not etc etc !!
DD, 13, stayed with a plain palette for colours, blue as a main contrast to cream, then mirrors, fancy shelves, neon lights with a colour change function (thanks Ikea) and a big chair (Ikea again) for reading in. I adore her room. A big book case, the reading chair and the make up shelf are all heavenly. It may not sound very exciting, but I'd have killed for that at her age.
What about a photo mural/ wallpaper on one wall? You can get lovely scenes of woodland / the planets/ city scape and pretty much anyhting you can think of. Or a Banksy inspired wall sticker (I've seen the one of the little girl looking up at her balloon floating off on NOTHS's website)
Forgot to mention...dd also takes photos. She's got her own digital camera and we've got framed pictures everywhere and she's got a canvas landscape print on her wall, that she made herself. Then she has lots of posters and a clock made from a record from her favourite band (thanks to a friends entrepreneurial little boy). I love the rock chic theme she's got going on. Wish I was 13 again....a little.