Different colors or such can server purposes such as easily distingusing user, or administration for others, and its only childish if the admin desgins it lame, and its not discriminating in any way. For example in pages and stuff unless you just know who someone is then you may not know they are a Admin, or maybe a mod. With such features you can quickly distinguish who someone is just from the color of their name.
I have a site I use it on and it serves a distinct purpose, and there are so many users that use it. Or you can look at just about every other CMS/Forum system out there, they have a default coloring scheme for default groups.
I have no plans to put default colors, just the feature admins to set HTML, and admin choice to allow users to set their own (something I would never do on my own site). Also this would allow for, plugins to use this feature as well.
Part of what your referring to is a typical "title" feature, most systems have, but the location and such can be used for similar things lol. Though I don't think we need to add "titles" since it can be added as extra fields.
The reason for needing to add support for HTML for users/groups is because the only way for such modifications are corehacks, and these things are corehacked enough by people that its something we should have. It will also be very non-invasive, and for those that don't want it they will never know it was even there.