Finder’s colour labels have been replaced by tags in OS X Mavericks. The new tag feature is a more flexible way to add meta data to a file, and you can add your own tag, rename it or change the associated color. Tag is a useful tool to help you to organise, filter and search for your files.
To add a new tag to a file, right-click on the file in Finder and select Tag… from the context menu. A tag drop-down window will appear to allow you to assign an existing tag or add a new tag.
To rename or change the color of a tag, go to Finder > Preferences and click on the Tags tab.
Finder by default lists down your recently used tags at the sidebar. This makes it quick and easy to assign tag simply by dragging one or more files to the tag at the sidebar.