\version "2.16.1"

gac = #(make-engraver
  (acknowledgers
   ((grob-interface engraver grob source-engraver)
    (let ((layer (ly:grob-property grob 'layer)))
      (ly:message "~A layer ~A" grob layer)
      (if (eq? 1 layer) (ly:grob-set-property! grob 'color red))
      ))
   ))

\layout {
  \context {
    \Score
    \consists #gac
    \override NoteHead #'layer = #(lambda (grob) 1)
  }
}

\relative c'' { c4 }