You may find a few of the examples in this box helpful. Add the code that you want to your MediaWiki:Wikia.css. Replace Some user name with the user's username and follow the instructions in the comments (this need to be applied manually on each certain user).
/* the user's username color above his messages */
body.ChatWindow section#WikiaPage > .Chat li[data-user="Some user name"] .username {
color: orange
}
/* the user's chat message color */
body.ChatWindow section#WikiaPage > .Chat li[data-user="Some user name"] .message {
color: orange;
}
/* the user's username color in the users rail */
body.ChatWindow #Rail li[data-user="Some user name"] .username {
}
Also, if you import the custom chat JS and CSS pages, it's musch easier and doesn't require the body.ChatWindow prefix (also doesn't mix chat code with other wiki code).