This tutorial is how to create a custom muzzle flash, or anything else that can be applied to this.
Download this base picture for the tutorial, or make your own that is similar../base.png
Open up the gradient in a Photoshop
Go to Images>Adjustments>Gradient Map...
Click on the gradient bar
You will see this:
Step 6: Tweak the gradient until you get something interesting.
Congratulations, you created a muzzle flash bitmap based off of the Halo 3 Plasma Rifle's Muzzle Flash.
You can make other gradients, to create different styles of muzzle flashes. Also other cloudy/puffy shapes can make it look different in shape.