Here's what I do: Base paint in Tamiya gold (spray can;Floquil's railroad line also has a nice brass paint that I will use when I'm not too lazy to load up the airbrush), apply a light coat or two of Tamiya clear orange and/or Tamiya clear smoke (with a brush, no real need to thin the clear paints). I will vary how many coats of each I apply, just play around until you get a finish you like. Then, to replicate the effects seen on spent shells, I will use pastels or weathering powders for the soot, discoloration, etc. These also help in dulling the shine that may be a bit over-the-top. Drybrushing in a dark grayish/blue also works if you want a more dramatic discoloration, as seen in the picture you posted.