\set fretLabels = \markuplist {"a" "b" \italic \smaller "c"}
<f d>4. <bes>8 <g e>4
\set fretLabels = \markuplist {\with-color #red "a"
\set fretLabels = \markuplist {"a" "b" \italic \smaller "c"}
<f d>4. <bes>8 <g e>4
\set fretLabels = \markuplist {\with-color #red "a"