Whew... so many opinions that see saw back and forth on this subject.
IMO I would say that depending on how long she was gone, maybe it was a little fast but maybe talking about it is the answer. It is YOUR house so you should be able to decorate how you want but understand her feelings that for 18 years, that was her space, her sanctuary where she could get away from life, her special place that she could decorate and express herself away from the rest of the family. I may not be happy with the things my children have done with their rooms but unless there is destruction of property, I consider it their territory and just shut the door. Was there a reason maybe that you felt the need to redecorate right away, was it maybe your way of dealing with her being gone? If it was discuss that with her, show her you still love her but express that it was hard for her to go and that was how you dealt with it. If it had nothing to do with that then explain what made you choose to do that. Also stress she is always welcome in your home (if she is) and no matter the color of paint that it will always be her room.