^It looks to me like the three chambers can be rotated, so I assume that's how you set the power level: whichever chamber is in line with the rifle barrel is the one that's generating the beam. I'm guessing green is stun, amber is kill, and red is disrupt/disintegrate.
rotate the chambers--counter-clockwise. He does it just as he delivers the line: "If you have not received a signal from me within twelve hours, you'll proceed at maximum warp to the nearest Earth base with my recommendation that this entire planet be subjected to a lethal concentration of neutron radiation. No protest on this, Mark. That's an order."