I think that I have found an answer:  in the \setupinteraction command that
manages URL's, hyperlinks, PDF properties, etc. include
color=,contrastcolor=,


\usemodule[letter]

%% Would like this URL to match the color of the return address ("location")
\useURL[author-email][mailto:author@myhome.com][][author@myhome.com]
%% Would like this URL to match the color of the recipient's address
("address")
\useURL[recipient-email][mailto:addressee@gmail.com][][addressee@gmail.com]
%% Would like this URL to match the color of the main body
\useURL[knuthbio][
http://en.wikipedia.org/wiki/Donald_Knuth][][http://en.wikipedia.org/wiki/Donald_Knuth
]

\setupinteraction[state=start,color=,contrastcolor=,style=normal]

\setupletter[
  name={Michael Ash},
  email={\from[author-email]}
  ]

\setupletter[signature={Michael Ash}]

\defineletterelement[layer][location][example]{\correspondenceparameter{name}
\correspondenceparameter{email}\par}
\setupletterlayer[location][alternative=example,color=red]

\defineletterelement[layer][address][example]{\correspondenceparameter{toname}\par}
\setupletterlayer[address][alternative=example,color=blue]

\starttext
\startletter[
  toname={The Addressee\\\from[recipient-email]},
  opening={Dear Addressee:},
  closing={Best regards,}]

  Enjoy \from[knuthbio]

\stopletter
\stoptext



On Tue, Jul 30, 2013 at 5:57 PM, Michael Ash <mash@econs.umass.edu> wrote:

> I would like to change the color of URL's in parts of a document.
>
> In particular I am using the letter module and would like to have the
> color of URL's match the color of text in the letterhead and then change to
> match the color in the main body of the letter.
>
> I can set the URL color once with \setupinteraction but I do not see how
> to change it thereafter.
>
> Thank you very much for your guidance.
>
> Best,
> Michael
>
>
>
> \usemodule[letter]
>
> \setupinteraction[state=start,color=black]
> %% \setupinteraction[state=start,color=red]
>
> %% Would like this URL to match the color of the return address
> ("location")
> \useURL[author-email][mailto:mash@econs.umass.edu][][mash@econs.umass.edu]
> %% Would like this URL to match the color of the recipient's address
> ("address")
> \useURL[recipient-email][mailto:addressee@gmail.com][][addressee@gmail.com
> ]
> %% Would like this URL to match the color of the main body
> \useURL[knuthbio][
> http://en.wikipedia.org/wiki/Donald_Knuth][][http://en.wikipedia.org/wiki/Donald_Knuth
> ]
>
> \setupletter[
>   name={Michael Ash},
>   email={\from[author-email]}
>   ]
>
> \setupletter[signature={Michael Ash}]
>
> \defineletterelement[layer][location][example]{\correspondenceparameter{name}
> \correspondenceparameter{email}\par}
> \setupletterlayer[location][alternative=example,color=red]
>
>
> \defineletterelement[layer][address][example]{\correspondenceparameter{toname}\par}
> \setupletterlayer[address][alternative=example,color=blue]
>
> \starttext
> \startletter[
>   toname={The Addressee\\\from[recipient-email]},
>   opening={Dear Addressee:},
>   closing={Best regards,}]
>
>   Enjoy \from[knuthbio]
>
> \stopletter
> \stoptext
>
>


-- 
Michael Ash, Chair, Department of Economics
Professor of Economics and Public Policy
University of Massachusetts Amherst
Amherst, MA 01003
Email mash@econs.umass.edu
Tel +1-413-545-2590 Twitter https://twitter.com/michaelaoash