Thank you, Wolfgang and Jean-Pierre!

I've modified your example and now it resembles what I was looking for.
Now I can use \inright to get annotations on the right of the main text,
but inside the width of the normal text.

The annotations are aligned to the line they occur in the main text.

It's somewhat tricky, because I need to leave the second paragraph empty.
Is there a way to obtain that effect without setupparagraphs?

Anyway it works as I wanted, so now I have a solution.
Thank you very much!

Best regards,
Massi

%% begin example
\setuplayout[rightmargin=7cm]

\definenarrower
  [SidenotesMargin]
  [before=\blank,
   after=\blank,
   right=-1cm,
   %right=\rightmarginwidth,
   default=-right]

\defineparagraphs[Sidenotes]
\setupparagraphs [Sidenotes]  [n=2,before={\startnarrow[SidenotesMargin]},after=\stopnarrow]
\setupparagraphs [Sidenotes:1][width=.55\localhsize,distance=.1\localhsize]
\setupparagraphs [Sidenotes:2][width=.35\localhsize]

\starttext

\input knuth

\startparagraphs[Sidenotes]
  \startparagraphscell
    A paragraph spanning some lines of text 
    with sidenotes that should stay aligned\inright{aligned}
    with the text they annotate;
    \input ward
    Some more words\inright{more words, 
      spanning more than one line on the margin} 
    to control whether the sidenotes are
    aligned with the text they annotate.
  \stopparagraphscell
  \startparagraphscell\stopparagraphscell
\stopparagraphs

\input knuth

\stoptext
%% end example

mf
30. September 2016 um 00:09

Thanks for the answer, Wolfgang.

That's not style and color, it's a sort of two-column layout with the
main text in the left column and annotations (comments) in the right
one.
This is not a real multcolumn text because in verbatim text each space
in the input appear also in the output and in the examples spaces
where used to align the code comments on the same position.
Example (it does not work):

\starttext

This is the main text\sidenote{annotation} flowing in the left
column...

\stoptext

resulting in

|This is the main text flowing   | annotation               |
|in the left column...           |                          |

You can do it with setuplayout and a wider right margin, using \inright
for annotations, but it's for the whole document or a whole page.
Is that possibile only for a section of text inside a page?
You can use the narrower environment to change the margins for a certain
text area and use the paragraphs environment to create two columns
with different widths in this area.

%% begin example
\definenarrower
  [SidenotesMargin]
  [before=\blank,
   after=\blank,
   right=\rightmarginwidth,
   default=-right]

\defineparagraphs[Sidenotes]
\setupparagraphs [Sidenotes]  [n=2,before={\startnarrow[SidenotesMargin]},after=\stopnarrow]
\setupparagraphs [Sidenotes:1][width=.6\localhsize,distance=.1\localhsize]
\setupparagraphs [Sidenotes:2][width=.3\localhsize]

\starttext

\input knuth

\startparagraphs[Sidenotes]
    \startparagraphscell
        \input knuth
    \stopparagraphscell
    \startparagraphscell
        \input ward
    \stopparagraphscell
\stopparagraphs

\input knuth

\stoptext
%% end example

Wolfgang