\version "2.22.1"

whitenote = \once \override NoteHead.style = #'petrucci
nohead = \once \override NoteHead.transparent = ##t
sstem = \once \override Stem.length = #4
rednote = {
  \once \override Stem.color = #red
  \once \override Flag.color = #red
  \once \override NoteHead.color = #red
}
mrg = \mergeDifferentlyHeadedOn % doesn't seem to do anything??

\score {
  \new Voice {
    \autoBeamOff
    \stemUp a' 
    <<{ \stemDown \nohead \sstem \rednote \mrg c''8 }\\{\stemUp \whitenote \rednote c''2*1/4}>>
    \stemUp 
    a'2
  }
  
  \layout{ 
    \context {
      \Voice
      \override NoteHead.style = #'blackpetrucci
      \override Flag.style = #'mensural
    }
  }
}