Visited tags are displayed with the link color, rather than the standard link appearance: tag-you-probably-havent-visited will be grey right now, but when you visit it, it'll turn gold here, or red on main site. If you clicked that too soon, try this-other-unique-one.
It doesn't show up this way in post previews, which are always just grey, regardless of visited-ness. That's how it used to be, so this changed during the redesign.
This affects the appearance when visiting a tag without a tag wiki:
Apparently this might just be a feature of the redesign though, since it occurs this way too on Meta Stack Exchange:
(usual tag appearance is grey)