

First, I paint the whole cameo with one soft background color. It hardly matters which color, usually a goldish, or faintly greenish, or mauvish. The portrait usually takes up the majority of the cameo anyway. The background color helps seal the plaster, so it doesn't keep absorbing the paint as I paint.
You can see two of my detail brushes here. Pretty small, and they get used up quickly. I'm always having to buy brushes, I'm tough on them.
Then I usually do a sketch in dark colors, hunter green, burnt umber, and maybe a little black.