--- a/mercurial/color.py	Tue Feb 21 17:50:04 2017 +0100
+++ b/mercurial/color.py	Tue Feb 21 17:51:43 2017 +0100
@@ -162,6 +162,8 @@
         _terminfo_params.clear()
 
 def _modesetup(ui, coloropt):
+    if ui.plain():
+        return None
     if coloropt == 'debug':
         return 'debug'