\version "2.21.0"
\paper {
  #(set-paper-size "a4")
  indent = 0\cm
  left-margin = 2\cm
  line-width = 17\cm
  top-margin = 2\cm
  bottom-margin = 2\cm
  ragged-right = ##f
  ragged-bottom = ##f
  ragged-last-bottom = ##f
  tagline = ##f
}

\markup { \postscript #"
        
    << /PageUsesTransparency true
       /CompatibilityLevel 1.4
       /PageSpotColors 0
    >> setpagedevice

    0 .pushpdf14devicefilter % depth .pushpdf14devicefilter -

    /rectfill {
        gsave 4 2 roll moveto 1 index 0 rlineto
        0 exch rlineto neg 0 rlineto closepath fill grestore
    } bind def

"}


\relative a' { \repeat unfold 12 { a1 4 4 4 4} \bar "|."} \addlyrics { foo }


\markup { \postscript #"
    
    grestore
    
    << >> clippath pathbbox newpath .begintransparencygroup

    1.0 .setshapealpha

    1.0 .setopacityalpha 1 0 0 setrgbcolor 30 0 20 -90 rectfill
    0.8 .setopacityalpha 0 1 0 setrgbcolor 40 0 20 -110 rectfill 
    0.6 .setopacityalpha 0 0 1 setrgbcolor 50 0 20 -130 rectfill 
    0.4 .setopacityalpha 1 0 0 setrgbcolor 60 0 20 -150 rectfill 
    0.2 .setopacityalpha 0 1 0 setrgbcolor 70 0 20 -170 rectfill
    
    0.6 .setshapealpha
    0.6 .setopacityalpha
    1 0.7 0 setrgbcolor 

    gsave

" }

\relative a' { \repeat unfold 12 { a1 4 4 4 4} \bar "|."} \addlyrics { foo }


\markup { \postscript #"

    .endtransparencygroup
    .poppdf14devicefilter

" }