Distinguish inactive windows

[Joe Riel]

I'm creating an application where focus is in a particular Emacs
frame, call it the control panel.  Events in the control panel affect
a selected window in a separate frame.  How can the selected window be
visually distinguished from the other windows in its frame?  I'd like
to, say, set mode-line background of the selected window to a
particular color.  Using (set-face-background 'mode-line-inactive
"yellow") doesn't work because that changes the mode-line of all
inactive windows, not just the one desired.  Similarly, setting
mode-line doesn't work because the active window is in the control
panel, not the one that is being changed.

-- 
Joe Riel