It's supposed to be a dirty or lived-in white. They tried to stay true to the book rather than the movie. Imagine if you went camping for a few weeks & never washed your white t-shirt; it would get some color to it.
True but there again I'm not a demi-god wizard. Any spellcaster who can zap Nazgul and Balrogs should have a little spell tucked away that can beat ground in stains and leave their clothes 'Persil' white.
Edit: Added commercial endorcement