I had this conversation with someone months ago. Here is my take on it. He does build the Mark III with a Gold Titanium Alloy, the thing is that the metal contrast itself is fictional. Theoretically it was from his Seraphim satellites. The thing is that a Gold Titanium alloy composite does exist in dentistry where ceramic bonding is required, I'm assuming this is the kind on bonding they are referring to and if you follow that idea you are dealing 90% gold/10% titanium mix where the damage itself would actually be a mix of the two colors but more grey or a bronze coloring than steel. I'm imagining from the descriptions given from the film itself that the gold coloring is a plating that goes over the titanium bond so you'd have layers of gold on the surface maybe even a layer or two underneath with the rest being the metallic steel coloring to the core.
Its a fictional compound so in reality you are dealing with a wild card in terms of accuracy but if I were to direct one I would say for the best example of damage the entire undersuit would be gold from head to toe with the "hot rod" red painted over it so a damage on the shoulder paneling for example would first be gold under the red and then metallic under that.
