Hi,
I put this in my layout block
\context {
\Score
\override CodaMark.color = #red
\override SegnoMark.color = #red
\override SectionLabel.color = #red
\override SectionLabel.font-size = #3
\override RehearsalMark.font-size = #3
\override RehearsalMark.color = #red
}
The result is large red rehearsal marks and large red section labels but black segno and coda signs. I looked through the internals documentation (thanks to Michael Werner for your help there).