Sorry for late reply lol.
Basically, those green usernames are PC Supporters, the members who donated money to keep PC running because without a donation system PC would still not be here today. You may have seen a few members with green names like mine, but despite their unique appearance they aren't actually staff hahaha.
Click here to learn more about PC Supporters and the donation system. Basically, they get additional features such as blogs, access to secret forums and also extra profile customization.
I got mine as a contest prize/gift and didn't necessarily donate, but a few other members obtained it in a similar way.