Helps determine conductor phasing and coloring based on circuit number, using the coloring scheme generally recommended for building wiring in the United States.
-120/240V: Black, Red, Blue
-277/480V: Brown, Orange, Yellow
This may vary from region to region, and care should be taken to determine correct wire identification procedures for your job. This has also varied over time, so do not trust the wire coloring to determine existing wiring phasing unless it is clearly marked.
While all electricians should be able to figure this out in their head, it's always nice to have a little help.