How to indicate window needs saving

[Paul Looney]

Bill,
I use a small (15 pixel) square button in the upper right corner of  
the window. A round button would work, too - and probably look better  
on OS X.

Green means the window is uptodate and does not need saving.
Yellow means there are unsaved changes (clicking on the button when  
Yellow will save the changes).
Red means the window is locked and no changes are possible.

In addition, when Green the button is labeled "O", when Yellow "!",  
when Red "X".

Paul Looney