Hi Hans,
To progress in the use of Metafun, I try to do the same cover page as your document 'Metafun ". Without looking at the code (but using the definitions of your colors), I managed to get the mosaic.
I would write the word "MATHÉMATIQUES" as you did with the word "CONTEXTEN".
Thank you,
Fabrice


\setuppagenumbering[location=]

\setupcolors
  [state=start]

\definecolor[maincolor][g=.6]
\definecolor[pagecolor][s=.6]
\definecolor[darkcolor][s=.2]
\definecolor[textcolor][r=.6]


\startMPinclusions
  color maincolor ; maincolor := \MPcolor{maincolor} ;
  color pagecolor ; pagecolor := \MPcolor{pagecolor} ;
  color darkcolor ; darkcolor := \MPcolor{darkcolor} ;
  color textcolor ; textcolor := \MPcolor{textcolor} ;
\stopMPinclusions

\startuseMPgraphic{CoverPage}
  path p ; p := OverlayBox xyscaled (1/2,1/7) ;
  fill OverlayBox withcolor pagecolor ;
  path q ; q := p enlarged -12pt randomized 12pt ;
  pair a ;
  for j=0 upto 6 :
     for i=0 upto 1 :
        a := (i*xpart urcorner p,j*ypart urcorner p) ;
        q := p enlarged -12pt randomized 12pt shifted a ;
        circular_shade
          (q, ceiling(uniformdeviate 4), maincolor, .5maincolor) ;
       draw q withpen pencircle scaled 8pt withcolor darkcolor ;
      endfor ;
    endfor ;
\stopuseMPgraphic

\defineoverlay[CoverPage][\useMPgraphic{CoverPage}]

\setupbackgrounds
  [rightpage]
  [background=CoverPage]

\starttext

\stoptext