Normally, I would say "a piece of white paper". "White" is more closely associated with "paper" than "piece" is.
Student or Learner
Here is a question that has bothered me for quite a long time. Should I say 'a white piece of paper' or 'a piece of white paper'? Are both expressions correct but having different meanings or only one of them is right?
I'd be more likely to use the first version to stress that I needed white paper, and not some other color.