This code changes the color of the admin and bureaucrat usernames and makes them bold (although it can be done for any user).
Implementation
Copy this into your wiki's MediaWiki:Wikia.css;
a[href$="/wiki/User:Admin's_Username"] {
color: #006600 !important;
font-weight: bold !important;
font-family: Arial !important;
}
Replace "Admin's_Username" with the admin's/bureaucrat's username. You can also change or remove the other values to change how the user's name will appear.
Repeat the code for each user's name you want to change the color of.
Optionally, you can also change the color for the Message Wall, User blog, and Contributions links, in addition to the user's name, with this:
a[href$=":Admin's_Username"],
a[href$="/Admin's_Username"] {
color: #006600 !important;
font-weight: bold !important;
font-family: Arial !important;
}