Good question. It's a useful but incomplete term. Just like white is incomplete. There are always things to be found more white.
This is pedantic and nonsensical, IMHO. White is well defined as "the lightest color" [0]. There is nothing "whiter than white" in the objective sense. It's also a very common topic in introductions to philosophy in order to define how one makes an apology.
[0] https://www.britannica.com/story/are-black-and-white-colors