I took the light gun test this past November, for I too allegedly have a color deficiency. If you can tell what color traffic lights are, you'll have no problem with the light signal test. An FAA examiner will meet you at the FSDO and will walk you or drive you to a point 1000 feet from the control tower. The FAA examiner will call the tower over the radio and ask them to flash the light gun at you. It may take a while for the controllers to figure out how to use the gun, since they don't use the gun very often. But the examiner will see the same lights you see, so don't sweat it if it takes a while for the lights to show up.
After you tell the examinor the color of the light flashes, you'll then move back to 1500 feet and they'll do the test again. When you pass the test, they'll hand you a new medical with the night restriction removed and they'll hand you a "Statement of Demonstrated Ability", (S.O.D.A.)., which you'll want to keep in a very safe place for as long as you intend to exercise pilot privledges.
Like you, I was nervous about taking the test, so I went to my local airport, (not the one where the F.S.D.O. is located and where I took the actual test) and took a self-administered practice test. I called the tower from my airplane radio and asked them to flash the light gun at me. They were more than happy to oblige, (of course, I called them on the phone first to let them know I was coming out and why I wanted them to shoot the light gun at me).