I discovered this by hunting through font-ini.mkii:

\setupbodyfontenvironment [default] [em={\slanted\color[red]}]

Seems to work well.  You see, I want to use the same ConTeXt files for two
purposes: to create a set of printable notes (without colour) and to make a
set of displayable onscreen notes (with colour).  This means that {\em this
text} appears slanted in the printed notes, and slanted and red onscreen.

Thanks very much!

-Alasdair

On Tue, Feb 21, 2012 at 5:47 PM, Aditya Mahajan <adityam@umich.edu> wrote:

> On Tue, 21 Feb 2012, Alasdair McAndrew wrote:
>
>  In some overheads I'm creating, I'd like emphasized text {\em like this}
>> to
>> appear as red.  I hoped that something like this would work:
>>
>> \setupbodyfontenvironment[**default][em={slanted, textcolor=red}]
>>
>> but it doesn't.  What is the canonical way of obtaining red, emphasized,
>> text?
>>
>
> \definehighlight[emph][style=**slanted, color=red]
>
> \emph{...}
>
> Aditya
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