The alt-l command toggles coloring of secondary layers in the graphic notation.
Pressing alt-l will toggle colorization of secondary layers in the notation. This feature can be useful for proof-reading music by highlighting unusual sub-spine/layer organization in the data that is not visible in the graphical notation, such as interchanged layers (cases where layers 1 and 2 should be interchanged).
Here is an example of using alt-l on a real piece of music:
