Though individual warriors sometimes use personal colors, the Irregulars standard scheme is blue for First Battalion and red for Second Battalion. The color starts on the right side of a unit and fades to black towards the left.
Per Mercenaries Supplemental, page 31.
Tarantula, Mercenaries Supplemental (color plates)
The Irregulars' insignia is a DropShip rising on a plume of silver stars. This is displayed on the left torso of 'Mechs, left wing of aerospace units, and to the left on the front of vehicles. Kills are recorded by painting small golden fish under the cockpit or on the turret. Per FM: Mercenaries Supplemental, page 31.