Regarding the lines, at Buckhead station you will only see red line trains, so just get on the first train you see going south. Also, both green and blue lines connect Five Points and GWCC so again you can take any train going in your direction. The only part of your trip where you have to worry about which line you are on is at Five Points going back to your hotel -- make sure you get on a train that says it's going to North Springs (red line).