copied from Ars Technica's review:
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.
The actual Tag information is stored in its own separate extended attribute named
com.apple.metadata:_kMDItemUserTags
. (As far as I know, this is a "normal" extended attribute, not a proxy for legacy HFS+ metadata like
com.apple.FinderInfo
and
com.apple.ResourceFork
.) In this example, the "Green" and "Red" Tags have been applied to the file
Hello
in that order.