Patrick,
I keep the brass pretty simple. I like to start by painting the trumpet in a fairly thin coat of gloss black - metallics look better painted over a glossy surface (not spray silver!). The base color is gold printer's ink (PI) with a little burnt umber oil paint added. Highlights with straight gold PI and shadows with burnt umber. I coat the whole with either Model Master semi-gloss or gloss clear acrylic depending on what seems right for the figure.
Bill