%% http://lsr.di.unimi.it/LSR/Item?u=1&id=889

%LSR thanks to David Kastrup for this code snippet.
%=> http://lilypond.1069038.n5.nabble.com/Catch-direction-operators-td151552i20.html

\paper { tagline = ##f }

colorizeDir =
#(define-music-function (item)
   (symbol-list-or-music?)
   (define (grob-colorize-dir grob)
     (let ((ev (event-cause grob)))
       (case (and ev (ly:event-property ev 'direction))
         ((1) red)
         ((-1) blue)
         (else '()))))
   ;; updaters note for 2.20:
   ;; \tweak was changed in 2.20, thus switching to \override
   ;; (\override works for 2.18 as well)
   #{ \override $item . color = #grob-colorize-dir #})

mapList =
#(define-music-function (fun lst)
   (ly:music-function? list?)
   #{ $@(map (lambda (s) #{ $fun $s #}) lst) #})

music =
{
  a2-( b)
  a^( b)
  a''_( b)

  c''-3\4-"xy"
  c''_3^\4^"xy"
  c''^3_\4_"xy"
}

% only color Slur and TextScript grobs
\new Voice \with { \mapList #colorizeDir Slur.TextScript }
{ \music }

% color all grobs
\new Voice \with { \mapList #colorizeDir #(map car all-grob-descriptions) }
{ \music }