Well, the class for that area is
.xenOverlay.memberCard .userInfo h4, so that might get you started. It's just a matter of making so a different color shows only for those with custom titles.
You may be able to do it with conditionals. It's not explicitly in the guide, but you may be able to get on the right path with
@Brogan's Conditional Statements resource.
I asked a similar question once, but that was regarding adding color to user titles in general. I vaguely remember
@Chris D answering that, but I can't find the thread.