Rob- continues to look fantastic! Some details if you wish to add more weathering:
Underneath would be black from dirt/grease/coal dust; a very popular paint for this with model RR's is Floquil Grimy Black. I've come to be quite fond of MM U.S. Army Helo Grey, which is not completely black but sort of an asphalt color.
Any fittings that move on the valve gear (connecting/drive rods for ex.) would be greased constantly, so some sort of gloss black would work. This also goes for the area behind the wheels on the frame.
The area immediately in front of the lower half of the cab is the firebox (right above the last driver wheel). This area gets intensely hot, obviously, and at night can be seen glowing a dull cherry red.
The boiler: this thing is full of water and steam under tremendous pressure. That means any of the vertical ribs on your boiler will have evidence of oxidation, or at the least calcium staining (grey/light grey streaks). This includes the riveted area toward the front of the boiler. That marks the separation of the boiler from the smokebox- lots of hot air and, well, you guessed it, but no water.
Steam drives the valve gear via the cylinders, hanging from the lower front end of the frame. Again, oxidation and calcium stains. Same for the steam dome on top of the boiler, and any joints on exterior pipes. There are two domes as you noticed; the other is the sand dome where the sanding pipes eminate from.
Your loco wouldn't have much soot on the top due to the smoke deflectors (your wind guides) that were designed to guide smoke up and away from the cab and the rest of the train. These devices were prominent on Santa Fe and New York Central RR passenger steam locos in the '40s and '50s.
Sorry if this is a bit long-winded but I hope there's something useful to you in here.
Dave