Lyluatex Reuse code: Stylesheet or newcommand

Question

I want to color all the stanza numbers of many hymns with red. I would like to keep the color definition in a stylesheet and only pass "the number and dot" of the stanza as a parameter.

I was not able to make it work with \newcommand\mystanza[1]{}. The stylesheet from the user manual is confusing.

verseOne = \lyricmode {
    %... n1-> n20 -> number of stanzas.  I want to avoid defining the color 20 times
    %%6
    \set stanza = \markup { \with-color #red 6.} %<-- I want this in a stylesheet passing the number as parameter
    %\myestanza{6.} %does not work if defined as \newcommand\myestanza[1]{...}
    Et ju -- be me ve -- ni -- re ad te:
    Ut cum Sanc -- tis tu -- is lau -- dem te
    in sae -- cu -- la 
    sae -- cu -- lo -- rum.
    A _ _ _ men
}

...

\new Voice = "one" {
    \musicOne %defined elsewhere
}

\new Lyrics \lyricsto "one" {   
    \verseOne
}

Answer 1

After reading and rereading 4.7.3 Style sheets, I found the following solution

I saved these 4 lines code in an external file myscore.ily

mystanz = 
#(define-music-function
(string)
(string?)
#{ \set stanza = \markup \with-color "red" #string #})

Then in the music file I included the file

\include myscore.ily
...
verseOne = \lyricmode {
            \mystanz "8."
            Et ju -- be me ve -- ni -- re ad te:
}

Now I am able to change the color for all stanzas in a single place

Answer 2

Your self-answer is correct. In a document made with LuaLaTeX + lyLuaTeX, there are two different engines at play: LilyPond for the music, and LuaLaTeX for the text. These are completely separate. When you do \begin{lilypond}...\end{lilypond}, the part in the ... is interpreted by LilyPond. In spite of the superficial similarity of LilyPond syntax with TeX syntax, essentially the fact that commands are introduced by a backslash, there is no link from TeX to LilyPond. A backslash-prefixed command does not get interpreted by TeX but is literally passed, with the backslash, to LilyPond. So while you try to define \myestanza as a TeX macro, its invocation \myestanza{1.} does not get expanded by TeX but is just seen by LilyPond as "\myestanza{1.}", and LilyPond doesn't know about a command called \myestanza.

As you have found out, the solution is to write LilyPond functions and not TeX macros. (You will also notice that LilyPond is a much more structured and clean language than TeX.)