Have milestones inherit colors from parent projects

Summary: Fixes T10737. I agree that this is a better behavior than always making them blue (boring).

Test Plan:
what

{F1208333}

wut

Reviewers: hach-que, chad

Reviewed By: chad

Maniphest Tasks: T10737

Differential Revision: https://secure.phabricator.com/D15643

src/applications/project/storage/PhabricatorProject.php

@@ -573,7 +573,7 @@ final class PhabricatorProject extends PhabricatorProjectDAO
 
   public function getDisplayColor() {
     if ($this->isMilestone()) {
-      return PhabricatorProjectIconSet::getDefaultColorKey();
+      return $this->getParentProject()->getColor();
     }
 
     return $this->getColor();