The Clone Trooper sergeant seen here is wearing plain white armor, the same as the general infantry. In the first-generation Clone Trooper uniforms, higher-ranked officers were denoted by colored stripes on the shoulders and arms of their armor as well as their helmets, with a series of colored dots on their chest. A sergeant should have light green coloring in those places.
When Cody and Rex engage the commando droids, they shoot the droids repeatedly but the droids are not damaged, causing the clones to note that the droids "have tough armor." Just moments earlier, Rex shot a droid (disguised as a clone trooper) once in the head, through a trooper helmet, and the droid was rendered inoperable. If the droids' armor plating was so tough that it can withstand repeated blasts to the body with no visible damage, it makes no sense for the head, containing the most vital parts of the droid, to be so vulnerable.