I have a pikeman's pot helmet made of cast aluminum that is a natural silver color. Are there any simple ways to give a darker gray or brown coloration?
It depends on if whether or not the aluminum is already protected by a finish of some kind (i.e., if it is anodized or painted) if the aluminum is simply mill, (i.e., unprotected) the aluminum will tarnish and darken all on it's own. If it is already protected with some sort of finish, your only option I am aware of is to have it anodized to a darker color or have it painted with a color containing either kynar, acrofluor, or fluoropon. Hope this helps.
Alumi-black(gun blueing) may work as well. We used it on some obscenely shiny aluminum maille.