Add Finder labels via drag-and-drop
Many of us use Finder labels to mark and more easily differentiate icons and files. There are several 'official' ways of assigning a label to a selected file: from the File menu, from the Action menu on the Finder toolbar, or from the the Finder’s contextual menu. But reader adipoos points out another way to assign labels that isn't so well known.
First, you need to sort your files by label; to do this, click the Arrange button in the Finder toolbar, then select Label from the list. (If you don’t see that button, choose View > Customize Toolbar and add it.) If you haven’t yet assigned labels to any of the files in the current directory, the Finder will sort them all into a single No Label category.
To make use of adipoos’s trick, you need to create a section for the label(s) you want to apply; to do that, you need to assign that label to at least one file. So, if you want to apply the Red label to a bunch of files, at least one file or folder must already be labeled Red.
To add a label to a file in the No Label section, just drag it from there into one of the sections labeled with a color; that label color will be applied immediately. Similarly, you can remove a label by dragging a file or folder from one of the colored sections into the No Label category.
If you’re in List, Column or Cover Flow view, you need to drag the file onto the label name at the top of its section. In Column view, the file’s color will change, but it won’t immediately move to the new section; if you navigate to another folder then back to the original, the file should appear in the correct section.