gray_to_spot.ps

%!Convert from DeviceGray to custom spot color
%Here we override two ps functions: setcolorspace and setcolor
%works with ps files made with pdftops (because pdftops generates ps files that use the two functions)
%won't work with ps files made with gs ps2write (because gs generates ps files that use setrgbcolor, setcmykcolor ans setgray)
%bitmap images will be colour inverted. Change /Decode [0 1] to /Decode [1 0] in the source ps file to fix the issue

%spotcolor begin
/spotcolor [/Separation (PANTONE 705 U) /DeviceCMYK{dup 0.9 mul exch dup 0.1 mul exch dup 0.5 mul exch 0.1 mul}] def
%spotcolor end
  
/oldsetcolor /setcolor load def
/setcolor {
    count
    %if there is 1 value in the stack, this is a gray value
    1 eq{
        %we have to invert the value because
        % "gray-value must be a number from 0 (black) to 1 (white)."
        % "the color components must be between 0 (none) to 1 (full)."
        %if there is 1 value in the stack, this is a gray value
        -1 mul % =-value
        1 add % =1-value
    }
    if
    oldsetcolor
} bind def

/oldsetcolorspace /setcolorspace load def
/setcolorspace {
  dup
  %if the color space is gray, change it to spotcolor!
  /DeviceGray eq
  {
  pop
  spotcolor
  }
  if

  oldsetcolorspace
} bind def