Coconut
  Boy writes:
  I
  was playing co-op with my friend, I was in the last outdoor area, and there
  were still a few enemies alive. Meanwhile my friend was walking through the
  hallway towards the exit, suddenly all of the snow covered mountains had a
  green and red gradient pillars on them almost as if it was christmas time on
  halo. I don't think this has ever been documented before, but if i come across
  it again i'll be sure to upload pics.
   
    This
    isn't quite the same as the glitch described above. Play through the level
    normally.  Bring both players inside to the hallway before the control
    room. Now send one player outside while one player watches the control
    panels. If the player inside notices the lights flicker, or the control
    panels change color while the other player is moving around outside, then
    you've found the glitch.  It isn't always in the same place, and it
    doesn't always work. The position of the player outside and the direction
    of his camera are very important.
    -FrogBlast