After Elle goes and gets her hair dyed at the top to make her hair lighter, she is in the classroom. Her roots are really dark, then a little while after that they are light!
I think it was the lighting, besides, Reese Witherspoon (Elle) is naturally blonde, so there would be no reason for her roots to be brown all the time.
OK, on the DVD in the special features section it talks about how they had to experiment with all different kinds of haircolors and every now and then they wouldn't be able to get them exactly right so maybe they filmed as much as they could with that haircolor, then just re-did them, and maybe she let her roots grow out a bit and then had them redone.
Ok have you ever thought that she could have been getting highlights or even streaks? Or it could have been the lighting. Also when she has just been to the bathroom she comes back into her bedroom and leaves the door ope. She talks on the phone and after she hangs up the phone she hears Vivian and her friend talking about the party. Her door is now closed. It didnt swing shut because you never heard a door shut. Also it was fully open.