Good question! I look forward to the answers!
I think one point that really sticks out is the impracticality of foster children making decorating decisions. You might be re-doing the room over and over and over again. That would become quite costly.
An idea that occurs to me would be to have a location where the child could really show her own flair. For instance, a blank bulletin board. She could decorate it with fabric, paper, art, photos, writings, etc. You could do a rather neutral room that was warm and inviting but not individualized. It would reinforce the message that she is in a safe place without imposing on her that it isn't her own.
You could also have a couple picture frames with neutral pictures that she could choose to put pictures of her family in. Then you wouldn't make a girl who doesn't have them feel bad (looking at empty frames) or a girl who does have them feel bad (not having frames to put them in).
Not having been in this situation, I can't comment on more, but I look forward to the responses from those who can!
ETA:
Here's an example of what I was meaning. (Mods, it goes to an Opera how-to page and is not selling anything.) Click on See Demonstration.
That one is actually a bit more personality than I would want but I had trouble finding a page that wasn't a commercial link. However, what I have in mind is more of a B&B type feel. Warm and inviting, and yet quite neutral and classical. Traditional. That's the word! A place from which you could build on if the time ever came.
Personally, I would be weary of using pink for the central color. It would stink if you had a child who despised the color. No other color (in its presence or lack thereof) seems to illicit the same type of response than the one from a girl who hates pink.