I'm not sure it this is possible but I use a graphical terminal emulator that has a translucent background so I can see the GUI windows behind it (in this case tinted a shade of blue). I want my status Line to have this as its background color.
I was able to find this useful example: http://jasonwryan.com/blog/2010/01/07/tmux-terminal-multiplexer/
And it used the color default.
so what I wanted was:
set status-bg default
EDIT: this might not work in newer versions of tmux, see other answers below