First of all, the Colossus and the Iceni are both plot device ships. It's impossible to tell what the capabilities of a Colossus or an Iceni REALLY are, because [V] never showed us a situation where they are not modified to suit an outcome that [V] wanted. So why is there a BFGreen that is never fired? Obviously, they designed it in case they needed the Colossus to have beams that blew things up really well. Perhaps they even, at an earlier stage, wanted the Colossus to blow up more than one Sathanas before succumbing to superior numbers, perhaps being destroyed by TWO Sathani at the bitter end rather than just one. However, all of this is simply speculation. I say again: It is impossible to know what a Colossus (or Iceni, for that matter) is "supposed" to be armed with, as we only saw one ship of that class and its armament changed as the mission designer saw fit to achieve the outcome they wanted.
I will shoot (with a BFGreen) the next person I see comparing, using FRED, the Colossus or Iceni VS. 5 other ships or a Sathanas or something in head to head beam-free-all combat.
P.S. This is NOT a Colossus thread. -.-