Mark the gradual feature rollout message magenta

Test: make atest, atest-dev
Change-Id: I8ba1429169f4df05365ff79602bdae3d5fa503d8

diff --git a/atest/rollout_control.py b/atest/rollout_control.py
index 01ed645..9496d14 100644
--- a/atest/rollout_control.py
+++ b/atest/rollout_control.py
@@ -22,6 +22,7 @@
 import logging
 import os
 from atest import atest_enum
+from atest import atest_utils
 from atest.metrics import metrics
 
 
@@ -169,7 +170,7 @@
       )
 
     if is_enabled and self._print_message:
-      print(self._print_message)
+      print(atest_utils.mark_magenta(self._print_message))
 
     return is_enabled