I used an airbrush and had my end piece spinning. Started with yellow, then moved to brown and then blue on the end.
Practice with anything you can rotate. I held mine in my lathe but if yours is part of the gun (it looks molded on) then make a metal or wooden fork you can tape the gun into and put it in a cordless drill on slow. You get a much nicer and uniform effect if you rotate the gun and hold the airbrush still.
If you dont have an airbrush, Get one. Mine cost £20 with its own electric pump. I wouldnt be without it now..... amazing.