a.mw-userlink[href$="/wiki/User:USERNAME"] {
   color: COLOUR;
}

Put the code in MediaWiki:Wikia.css, change the USERNAME to any username you want, change the COLOUR to any colour you want, you could also add other CSS properties

Thank you!

Are you just asking about the colors, or also about how to give rights?

Also, I think the code will work better in this form:

a[href$=":USERNAME"] {
   color: COLOUR !important;
}

And usernames with spaces or special characters will need encoding.

Yes, please clarify what you mean by "make staff roles". You can make a lot of cosmetic changes but you can't create actual custom user groups that come with permissions.

Sorry, I did mean change the colors. I got it to work. Thank you all for your help!