How does Mavericks's Finder store tags?

Mavericks saves tags as extended attribute

Now that the NDA is lifted: Mavericks saves tags as an extended attribute, in com.apple.metadata:_kMDItemUserTags. You can check them yourself by using the mdls command like this:

mdls -name kMDItemUserTags Hello

John Siracusa's epic review of OS X 10.9 describes the tag architecture in some detail.

Tags are stored in an extended attribute named com.apple.metadata:_kMDItemUserTags. Its value is a binary property list that contains a single array of strings:

$ xattr -p com.apple.metadata:_kMDItemUserTags file3|xxd -r -p|plutil -convert xml1 - -o -
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<array>
    <string>Red
6</string>
    <string>aa</string>
    <string>Orange
7</string>
    <string>Yellow
5</string>
    <string>Green
2</string>
    <string>Blue
4</string>
    <string>Purple
3</string>
    <string>Gray
1</string>
</array>
</plist>

The tags for colors have values like Red\n6 (where \n is a linefeed).

You can use xattr to copy the tags from one file to another:

xattr -wx com.apple.metadata:_kMDItemUserTags "$(xattr -px com.apple.metadata:_kMDItemUserTags file1)" file2
xattr -wx com.apple.FinderInfo "$(xattr -px com.apple.FinderInfo file1)" file2

If the kColor flag in com.apple.FinderInfo is unset, Finder doesn't show the circles for colors next to files. If the kColor flag is set to orange and the file has the red tag, Finder shows both red and orange circles. You can set the kColor flag with AppleScript:

xattr -w com.apple.metadata:_kMDItemUserTags '("Red\n6","new tag")' ~/desktop/file4"
osascript -e 'tell application "Finder" to set label index of file "file4" of desktop to item 1 of {2, 1, 3, 6, 4, 5, 7}'

'("Red\n6","new tag")' is old-style plist syntax for this:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<array>
    <string>Red
6</string>
    <string>new tag</string>
</array>
</plist>

xattr -p com.apple.FinderInfo file|head -n1|cut -c28-29 prints the value of the bits used for the kColor flag. Red is C, orange is E, yellow is A, green is 4, blue is 8, magenta is 6, and gray is 2. The flag that would add 1 to the values is not used in OS X.