Well there is certain data that is only available to certain templates. What makes the normal messages work for your background colors is this bit of code. It adds a class you can target with CSS which I assume you've done already.
Code:
{xen:if '{$message.is_staff}', 'staff'}
Using that code in the template that generates the user info by the quick reply is the message_user_info template. That data for that conditional above isn't passed to the message_user_info template. Make more sense?