I like how your chipping came out. You highlight something I've been thinking about recently, in weathering a couple of WWII vehicles. Where the paint has chipped, the metal would discolor, and eventually rust/oxidize. I've been looking at metal objects, like tools, and metal scaffolding, wheel barrows, chipped finishes on cars, etc, to observe what the metal looks like. Often guys will use a silver or aluminum color for chipping, and sometimes it's a match (like on an aircraft with aluminum skin), but unless the area is continuously worn, rubbing off oxidized metal, it goes dull and most metals will have a grayish color. Your cart is a proof of concept for me.