Serious showpeople wash beards after every meal and dry them after every drink possible. A waterless shampoo like Self Rinse is easy- spritz on, towel through and off.
With a white dog nine tenths of the battle is to prevent the stain in the first place. Drinking from a water bottle instead of out of a bowl will do wonders, and is my number one recommendation. Eliminate foods and treats with dye or beet pulp.
Once it is set it is next to impossible to remove- there are 100 concoctions from Magic Touch+ peroxide to chalk+ peroxide cream to milk of magnesia+ peroxide. Many folks use equine grooming products with heavy duty stain removers.
I would consult with the dogs breeder to find out what they recommend.