[Damian]

You can use HTML.

For example to change the Administrator username color and style to red, bold and italic.

AdminCP -> Usergroups -> Usergroup Manger -> Edit user group Administrator(id:6) -> Username HTML Markup ->
Add to first box:

<span style="font-weight: bold; font-style: italic; color: #FF0000">

Add to second box:

</span>

Alternatively, or if you user more than 1 style and would like the username different per style:

Add a new CSS definition to your style,

.username_admin {
color: #FF0000;
font-weight: bold;
font-style: italic;
}

For Username HTML Markup, use:

<span class="username_admin">