Hello Dunny who got to the name first. Been using Dunnyrail since G scale Mad days. Hope you are all well under there. As for painting puffers I think 2 part ones tend to be used quite a bit, though I think some Automotive Colours are suitable for warm/hot locations.
You will hear lots of suggestions about hi-temperature automotive engine paints, which work. Personally, I found that regular acrylic paint from the craft/hobby shop showed no signs of problems when I painted the boiler of my Accucraft EBT #12.
Brass is very difficult, and if you are trying to make the detail parts black, then a chemical blackener might be easiest.