I use song colors to indicate the "feel" of each song, for easy putting together of setlists without, for example, too many soft/slow ballads in a row, or two many upbeat/rock/danceable songs in a row. For me, for example, green = slow/ballad, yellow = a bit more upbeat but still chill, orange = pop/more swingy but not yet fast or loud, red = uptempo/loud/danceable.
When shoving songs around to make up a list, the colorings are a HUGE help. I'd love to also be able to use dual colors (for example, type color = green PLUS background color of the field = X) so as to denote songs with, say, harmonica accompaniment by me. That way, I could more easily put sets together without having too many songs with harp in a row - or too few. It helps to make a set more diverse and interesting. Same could be done for my duo, for songs in which bass (instead of a 2nd guitar) is played. Or songs in which a BeatBuddy (drumkit) comes into play. In the case of bass, for example, it is desirable to group a few songs with bass so as not to have to be swapping instruments too many times in a row (e.g. from acoustic guitar to bass and back again). Instead, you can set up groups of 3-4 songs with bass, and then go back to more acoustic set.
Of course, I could use a single color for each of these special categories, but it would be great to keep my overall color system as-is and just add a secondary color for the instrumentation aspects.