[despe]

Colouring would be really useful.
Depending on the content of certain fields as proposed might be useful for some users.
For me it would be enough if I could set a background colour manually per song, e.g. in the song editor. With multi-select (existing multi-select mode works great!) it would be easy enough to use.
The background colour should be applied to the whole song title line (or title line and caption line or independent colours for title and caption) wherever the song shows up in a list within the library: setlists, songs, albums, collections...

I prefer colours to be binded to some property or parameter so that colours could be managed by changing the setting for property instead of multiple songs.
 
I like the idea of colour for titleline and colour being universal.