Colin, near the beginning of this episode, gets out of the lake and shakes his wet hair onto Amy and stretches. He clearly has dark underarm hair. In the last scene of the same episode depicting the same day, he is lying on his back with his arm over his head and has no underarm hair at all.
At 7:22, as Brian is talking about how the slices were wrapped for travel, the shadow of the camera and boom-mic are visible on his coat, just under his left arm.