color: show mode warning based on ui.formatted

ui.interactive is only for input and ui.formatted is for output.

hgext/color.py

@@ -275,8 +275,8 b' def _modesetup(ui, coloropt):'
 
     def modewarn():
         # only warn if color.mode was explicitly set and we're in
-        # an interactive terminal
-        if mode == realmode and ui.interactive():
+        # a formatted terminal
+        if mode == realmode and ui.formatted():
             ui.warn(_('warning: failed to set color mode to %s\n') % mode)
 
     if realmode == 'win32':