They have to be bright white to begin with. If they have freckles of any sort they will never look as white as if they were unfreckled.
Prevention is the key. Once you get stains, they are hard to remove.
Some dogs are very clean about themselves, others not so much.
I have yet to find a product that truly makes stained costs bright white again and I have tried everything.
All powder does is make a mess. It does take some of the glare of staining away, but the stains are still evident.
I swear (only by observation, don't ask for clinical trial reports) that some coat types are more difficult than others to keep clean.
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