What do cavities appear as on a dogs tooth from the human perspective?

I stopped brushing my dogs teeth for a few months while I settled into my new Berlin apartment and realized her teeth were getting pretty yellowed. I brushed her teeth for a week and the yellow is gone from all but her back molars. On one side there is a small black spot on the front side of the tooth. It has a smooth, gradient-like edge, not sharp or of high-contrast to the rest of the tooth at all. It's not large, probably the size of a ballpoint pen's point. Is this what a cavity (usually) appears like? Bonus question, how to remove yellow, high build-up of plaque? What kind of instrument is required? Guten tag.