coloring notes with more voices in a staff

[Go77]

Hi,

I'm trying to color the note heads of a single voice in a staff. I tried 
to do that with the following code:

/\version "2.24.3"
\relative c
  {
       f'2( g4.)
        << { d8 | f4 f4 g16 a }
           { \override NoteHead.color = #red
             g8 | d4 d4 e16 f
             \revert NoteHead.color } >>
        a8 r4
  } /

This results in the noteheads of /both /voices being colored instead of 
only the upper one (in this case):

So, is there any way to achieve my goal of coloring only the upper notes?

Joop