Unfortunately, the nature of denture teeth makes the neither permanently discolor or to be lightened.  Porcelain teeth are similar to the dish in your china cabinet.   While acrylic teeth also are made of a particular color of solid or layered acrylic.
It may have a surface stain or discoloration, but the overall color can’t be made any lighter than it was originally made to be.
The best and easiest way to have whiter teeth is to when they are being made to suggest or request to the practitioner that you would like a lighter shade.
It is simple to do at that time to your complete satisfaction.
Thank you