A. Dark and brown teeth are covered with tartar and possibly stains from the coloring or dyes put in dog food.
I would recommend brushing your pets teeth daily and also giving her a higher quality food which won't stain the teeth so readily. Also feeding her a dry kibble instead of wet food will help the teeth as well.
If there is staining there this in most cases won't be removed completely but getting your pets teeth cleaned and fluoride treated will remove the tartar and help keep your pets dental hygiene up to par.
A. I am unsure if you are describing tartar or if perhaps the tooth is discolored because the root is dead. Regardless you should bring your dog for a dental. Cleaning and dental radiographs will cover all bases and ensure good dental health which is very important.
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