Haha, everyone who can post in the general forum is a registered member. :D Right now we are going through a voting to change the colors of the member groups, so things are a little wonky compared to the FAQ.
Dark blue is the general registered members (formerly green), while orange (soon to be hot pink) is for new members.
Light grey is for lurker members who haven't posted in 30 days, and dark grey is for inactive members who haven't logged in for 30 days.
Everything from here on out will be changed over the next several weeks.
Donating members are pale green (was this your confusion?), staff members are... Well, the color I have. Is that some shade of pink? ;p Community Volunteers are light blue, and administration is red.
So, moral of the story, no need to do anything! The moment you created your account, you became a registered member.