I haven't found the right technique yet painting p2 helmets. P2 is my least favorite helmet to paint, for right now though. But what helped me is, paint whole helmet in a good shiny copper and lay on the patinas. Like start with copper, browns, rust then go with the greens.
Google some pictures of copper, some pictures show what copper does in the elements. Some show other colors depending on what the environment the copper is in.