Adjust email diff colors

Bug: Issue 6471
Change-Id: I8b2a60837350b741f0a77fe04acc90f42893c563
diff --git a/gerrit-server/src/main/resources/com/google/gerrit/server/mail/Private.soy b/gerrit-server/src/main/resources/com/google/gerrit/server/mail/Private.soy
index 556191d..967f9e8 100644
--- a/gerrit-server/src/main/resources/com/google/gerrit/server/mail/Private.soy
+++ b/gerrit-server/src/main/resources/com/google/gerrit/server/mail/Private.soy
@@ -92,11 +92,11 @@
  */
 {template .UnifiedDiff private="true" autoescape="strict" kind="html"}
   {let $addStyle kind="css"}
-    color: green;
+    color: hsl(120, 100%, 40%);
   {/let}
 
   {let $removeStyle kind="css"}
-    color: red;
+    color: hsl(0, 100%, 40%);
   {/let}
 
   {let $preStyle kind="css"}