In Turok 2, the Charge Dart Rifle has different strengths to its shots depeding on how long you hold the Z button. Also, the longer you hold the button, the straighter and farther the shots will go. Here is how long you have to hold the Z button to attain the different levels.

Red - Shocks the enemy for approx. 5 seconds. Tap the Z button.
Orange - Shocks the enemy for approx. 10 seconds. Hold the Z button for about half a second.
Yellow - Shocks the enemy for approx. 15 seconds. Hold the Z button for about a second before releasing.
Green - Shocks the ememy for approx. 20 seconds. Hold the Z button for 2 seconds.
Blue - Shocks the enemy for approx 25 seconds. Hold the Z button for about 3 seconds to attain this shot. *NOTE* This takes 2 away from your ammo instead of the usual 1.
Purple - Extremely straight shot that shocks the enemy for 30 seconds. Hold Z for more than 5 seconds to get the gun to shoot this. *NOTE* This takes 3 away from you ammo instead of the usual 1.