I have finally got all of the weapons on the Colossus. I had to do some re-modelling of the side towers but they're on now. Below are photos of the weapon positions.
Observation/AA Deck - The top deck features the two Thunderfire Anti-Aircraft guns.
Side - Defensive weapons, Battle Cannons and Twin-Linked Heavy Bolters (Heavy Ion Blasters on Demiurg Colossus). Same on other side.
Side - More Twin-Linked Heavy Bolters (Heavy Ion Blasters on Demiurg Colossus). Same on other side.
Side - The top of the tower has another Battle Cannon and Twin-Linked Heavy Bolter (added after the photo was taken). Same on other side.
Front - Thunder Cannon and more Twin-Linked Heavy Bolters (there's 16 in total)
Main Weapon - One of the most devastating battlefield weapons in existence; The Doomsday Cannon
Landing Pad - The landing pad features the silo doors for the Colossus' 4 Plasma Missiles