One of several new, pro-user friendly new features in OS X Mavericks is Finder Tags. On the surface, they work like any other tagging system, allowing you to categorize and later find files by association. But they also have quirks and oddities all their own. The reason for that can be found in how OS X handles metadata, both now and in the past. John Siracusa explains it like only John Siracusa can. Ars Technica:
When a Tag that has a color assigned to it is applied to a file in Mavericks, Label information is written to the Finder Info field exactly as described above, making Tags at least partly backward-compatible with pre-Mavericks systems. As more colored Tags are added to the file in Mavericks, the Label information changes to reflect the most recently applied Tag. If a Tag does not have a color assigned to it, the Label information is not modified.
There's a ton more to it - including HEX! - and Siracusa dives deep into not only the implementation, but its history. Like many of Apple's decisions, it's pragmatic. Superficial limitations and annoyances make more sense when given context.
Of course, the entire review is well worth reading, as it is every year.
Source: Ars Technica