You've broken your song
Just kidding. Does it say that you can't rename it because it's in use? This can happen if someone's uploading it from you at the time, or if you still have the song in your incomplete folder. If it's all downloaded & you're happy with it, delete it from the incompletes (if it's there) & it should work. Or try refreshing your library.

Normally, songs in red aren't being shared. But... you should still be able to rename it. To share a file, right click on it & choose Share File. Now it'll go black.Try renaming it now.

Hope I haven't confused you too much