I know why questions, answers and some other aspects of the site are color-coded and what they mean but I can't find why badges are color-coded. I'll include a snapshot to show what I mean.
Tag badges appear in white. When you earn enough upvotes and answers in a given tag you earn one of those badges (100 up and 20 answers, for bronze, 400 up and 80 answers for silver, 1000 up and 200 answers for gold). General badges appear in black.