Pass correct value to git color based on args.color

pre_commit/commands/run.py

@@ -224,9 +224,11 @@ def _run_hooks(config, hooks, args, environ):
                 '`pre-commit install`.',
             )
         output.write_line('All changes made by hooks:')
+        # args.color is a boolean.
+        # See user_color function in color.py
         subprocess.call((
             'git', '--no-pager', 'diff', '--no-ext-diff',
-            '--color={}'.format(args.color),
+            '--color={}'.format({True: 'always', False: 'never'}[args.color]),
         ))
 
     return retval

tests/commands/run_test.py

@@ -184,7 +184,13 @@ def test_global_exclude(cap_out, store, tempdir_factory):
         (
             {
                 'show_diff_on_failure': True,
-                'color': 'auto',
+            },
+            b'All changes made by hooks:',
+        ),
+        (
+            {
+                'show_diff_on_failure': True,
+                'color': True,
             },
             b'All changes made by hooks:',
         ),
@@ -192,7 +198,6 @@ def test_global_exclude(cap_out, store, tempdir_factory):
             {
                 'show_diff_on_failure': True,
                 'all_files': True,
-                'color': 'auto',
             },
             b'reproduce locally with: pre-commit run --all-files',
         ),