Patina is rust that has been smoothed over by handling.
Place your hammer in a sweat box or a steamy shower for a couple of days, or longer, and surface rust the part to the desired color, then hand wax gently to preserve the rust color and smooth out the surface.
If you need it darker or lighter, just remove the wax with acetone and re-rust until you get the right color match.
Faux patina kits are also available I believe, but these use a mild acid first I think, then surface rust, which is what the antique forgers use..?...
