"Boldness" is set separately from "foreground color", but it is not supported by Mutt. All you can do in Mutt is select a foreground and background color, no boldness.
In other cases, where it is possible to output arbitrary ANSI escape sequences, you would set a "bold color 208" as follows (you can execute that in any terminal that allows bold text to see the effect, Gnome Terminal works fine if you have it configured to display bold text):
echo this is NORMAL
echo '\e[1m' this is BOLD '\e[0m'
echo '\e[1;38;5;208m' this is BOLD 208 '\e[0m'
echo '\e[38;5;208m' this is NORMAL 208 '\e[0m'
(The SGR parameter "1" will turn on bold. You send it with
<Esc>[1m, and you can of course combine it with others, e.g.
<Esc>[1;5m or as in the above example, where
38;5;x selects a color from the XTerm 256-color space.)