Bungie.net : Halo 3 File Details I don't know how these people are making these types of effects but if any of you people from FH know maybe an explanation or something?
I think its on blackout. What they do is they spell out a word in the air and look up towards it. They put on filters to make the colors and look up at it and take the screenshot.
There alot of different effects that can be made by using certain weapons and filters on the blackout sky, this is one of them.