I have my own mybb3 forums as well so I can help you. Where it says this: <span style="color: green;"><strong>{username}</strong></span>
Replace where it says: color: with any color you want. At first your will have numbers and capital letters thats why you change it. The Admin one is
different. You would have to copy the mod thing (<span style="color: green;"><strong>{username}</strong></span>) and replace the color with whatever
you want. If your still confused plz post I will figure it out.
In the Admin CP go to Users and Groups>Groups>Administrator(Or mod whatever your doing)and then it says Username Style then
just do what I said in the post before.