I totally agree with Derek on the humbrol enamel, especially in the lighter usual colours for model steam, red, maroon or green, they soon get a dark sheen on them. But I have found that a dark blue stays almost at the same colour, and doesn't look out of place. In fact I used it on my puffin for over ten years, and it still has the original coat on it. But really for max effect, you cannot beat black paintwork and polished brass, a classic and regal combination.
Sperex do a range of coloured car engine sprays that are usually good for for up to 600 deg F. I have used that on copper boilers to very good effect, but only in satin black.
Painting brass is a real PITA, even with a blasted or etched finish. It will chip or peel at the first opportunity it gets. Bronze on the other hand (what most steam fittings should be made of) I have no trouble with, except for the dark colour change. Maybe it has something to do with the higher copper or lower zinc content.
I have a friend who can get consistent sticking results on brass with a light dusting of car etch primer, but I have always been unsuccessful. Maybe I was using too heavy a coat, or just maybe he is holding back on a secret technique bit.
John