Colorizing Header Fields
by Brent Welch

A set of resources specifies colors and other font attributes of a message header.

This is a list of mail header fields which have special looks when displayed in the message area. For each tagname, you should define another resource named m_tagname (for example, m_subject). This m_tagname resource has the text tag configuration options for its header field. You'll find the tag configuration options in the manual page for the Tk text widget. Two special tagnames are used for defaults if there isn't a more specific matching tag name: "default" applies to displayed fields and "hidden" applies to hidden fields (fields that have been scrolled off the top).
For example, here's what I have in my ~/.exmh-defaults file:
    *m_tagnames:            hidden subject from x-filters-matched
    *m_hidden:              -font 6x10
    *m_default:             -foreground black
    *m_subject:             -foreground blue
    *m_x-filters-matched:   -foreground blue
    *m_from:                -foreground "medium sea green"