When you start with the silver base coat (as I see you did) get a can of rubber cement. You can also use liquid latex (that's what I did) and I've heard of some people using mustard but have never used it myself. Apply the cement to the silver base coat wherever you'd like the top colored layer to chip away and show the silver under it. Once that's done and you are happy with where you applied the rubber cement then lay down the color. Wait until the paint dries enough so that you can touch it without leaving marks and gently rub the spots where the cement is. If you put the cement on thick enough it should rub away colored areas only where the cement was. Flat clear coat looks good on a weathered helmet, I think. Hope it helps.