The following code is an example of how to draw a graphic on every page of a document. The graphic (a grey rectangle) appears in a different place for each chapter, and the chapter number appears in the rectangle as well. This is often called a thumb index - your thumb can flip the pages of a book while you watch the contents flash past...
\stoptext
</texcode>
<context>
% by Hans Hagen, of course :-)
\unprotected \def\rawsectionnumber#1{\countervalue{\??se#1}} % will be in kernel
\setuppagenumbering
[alternative=doublesided]
\setupcolors
[state=start]
\setupbackgrounds
[page]
[background=thumbs]
\defineoverlay
[thumbs]
[\setups{thumbs}]
\def\NOfChapters{4} % i need to make this one available