%!
/oldsetrgbcolor /setrgbcolor load def
/setrgbcolor {
(in replacement setrgbcolor\n) print
                                %% R G B
  1 index 1 index       %% R G B G B
  eq {                  %%
     2 index 1 index    %% R G B R B
     eq {
                        %% Here if R = G = B
      pop pop           %% remove two values
      % setgray % "replace the 'setgray' with":
      0 0 0 4 -1 roll % setcmykcolor
      -1 mul          %% obtain -R on top of stack
      1 add           %% obtain 1-R on top of stack
      setcmykcolor    %% now set(cmykcolor) K (as 1-R)
     } {
       oldsetrgbcolor   %% set the RGB values
     } ifelse
  }{
    oldsetrgbcolor      %% Set the RGB values
  }ifelse

} bind def
/oldsetgray /setgray load def
/setgray {
(in replacement setgray\n) print
  % == % debug: pop last element and print it
  % here we're at a gray value;
  % http://www.tailrecursive.org/postscript/operators.html#setcymkcolor
  % setgray: "gray-value must be a number from 0 (black) to 1 (white)."
  % setcymkcolor: "The components must be between 0 (none) to 1 (full)."
  % so convert here again:
  0 0 0 4 -1 roll % push CMY:000 after Gray and roll down,
                  % so top of stack becomes
                  % ...:C:M:Y:Gray
  -1 mul          %% obtain -Gray on top of stack
  1 add           %% obtain 1-Gray on top of stack
  setcmykcolor    %% now set(cmykcolor) K (as 1-Gray)
} bind def


%~ # test: rgb2gray
%~ gs -dNOPAUSE -dBATCH -sDEVICE=ps2write -sOutputFile=./blah-slide-hackRGB-gray.ps ./HackRGB.ps ./blah-slide-gsps2w.ps
%~ # gray2cmyk
%~ gs -dNOPAUSE -dBATCH -sDEVICE=pdfwrite -sOutputFile=./blah-slide-hackRGB-gray-ci.pdf ./HackRGB-cmyk-inv.ps ./blah-slide-hackRGB-gray.ps
%~ # check separations - looks OK
%~ gs -sDEVICE=tiffsep -dNOPAUSE -dBATCH -dSAFER -dFirstPage=1 -dLastPage=1 -sOutputFile=p%02d.tif blah-slide-hackRGB-gray-ci.pdf && eog p01.tif 2>/dev/null