Re:Should I be worried?
In my own experience the reason the arm gets darker when she is cold, right out of the bath, crying, active, etc, etc. I because it fills with blood and gets larger. After she calms down it goes back to normal. This happens to me PWS on my face all the time. It does not hurt, I really can not tell the difference exept when I look in the mirror or someones says something at the gym when I am working out.
You do have the option of doing laser treatment. I do not think it really hurts but I did it for 14 years. Before they could not do any more for me because of the depth of my marks. I also have hemangiomas which I will probably never get removed.
From how you discribed your daughters PWS it does not sound every bad at all because of how light it is, But I am not a doctor, and everybody's marks are different in some way.
If you go to the left of this page and under resources click on Find a physician and then go to your state or one close to you and find a doctor. It does not hurt just to get it looked at. If you do decide to do the treatment aske the doctor about a V-beam Pulse Dye Laser. This machine is safe from even infents. I have used this machine myself and out of all the machines I have tried I recommend that one is the best and hurts the least.
I wish you the best of luck and if you have any other questions please ask away. If I can not answer it I am sure someone on this site can.