I have noticed that there seem to be two different types of gold badge, one filled in, the other, not:
I haven't been able to read what the differences are on this site.
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The dark ones are badges for certain accomplishments or milestones, like belonging to the site for a year (Yearling) or editing a certain number of posts (Copy Editor). The light ones are tag-related. For example, you'd get a 'Great Answer' badge by getting 100 upvotes on an answer, and you'd get a gold 'delphi' badge by getting a total of 1000 upvotes on at least 200 answers to questions that carry the 'delphi' tag.
Click on the Badges button at the top of this page to see the criteria associated with each type of badge. You can switch between the 'general' and 'tags' tabs to see the different badges of each type.