To find files by label with Spotlight, or in the Finder's find box, type label:1 (or any other number up to 7). The numbers don't seem to correspond exactly to the order in the Finder; but you can see them by doing mdls on files. Here's how each number corresponds to one of the Finder's label colors:
- 0 - None
- 1 - Gray
- 2 - Green
- 3 - Purple
- 4 - Blue
- 5 - Yellow
- 6 - Red
- 7 - Orange

