diff --git a/tool/lib/output.rb b/tool/lib/output.rb
index 25c6b47397..5c645daca6 100644
--- a/tool/lib/output.rb
+++ b/tool/lib/output.rb
@@ -10,12 +10,17 @@ class Output
     @vpath = VPath.new
   end
 
+  COLOR_WHEN = {
+    'always' => true, 'auto' => nil, 'never' => false,
+    nil => true, false => false,
+  }
+
   def def_options(opt)
     opt.separator("  Output common options:")
     opt.on('-o', '--output=PATH') {|v| @path = v}
     opt.on('-t', '--timestamp[=PATH]') {|v| @timestamp = v || true}
     opt.on('-c', '--[no-]if-change') {|v| @ifchange = v}
-    opt.on('--[no-]color') {|v| @color = v}
+    opt.on('--[no-]color=[WHEN]', COLOR_WHEN.keys) {|v| @color = COLOR_WHEN[v]}
     opt.on('--[no-]create-only') {|v| @create_only = v}
     opt.on('--[no-]overwrite') {|v| @overwrite = v}
     @vpath.def_options(opt)