replacing "31" with RED constant

tests/lint/trailing_whitespace.py

@@ -1,14 +1,14 @@
 import itertools
 import re
 
-from tests.util import colon, color, display_path, paths, Failure, Success
+from tests.util import colon, color, display_path, paths, Failure, Success, RED
 
 
 def display_trailing_whitespace(whitespace):
     # Show trailing whitespace with printing characters and in red
     # To make it easy to see what needs to be removed
     replaced = whitespace.replace(' ', '-').replace('\t', r'\t')
-    return color(31, replaced)
+    return color(RED, replaced)
 
 
 def display_failure(failure):