Select a layer or an item and pick a color. Or better yet, select bunch of layers and apply same label color to all of them. If the selected layer has a source, or an item is used in composition, you can set the same label to both of them at once.

Filter button lets you quickly see witch labels are used in the current composition or the project panel. Alt+click will filter unused ones.

Use mouse and keyboard combination to select a group of layers or items that have same label color. With a different keyboard combo you can append to selection, select everything except current label color or remove label from selection.

Home Page - http://aescripts.com/labels/