Expl3: Expansion of mappings over lists

Question

The package xcolor can "prepare" color definitions in such a way that they're only loaded if they're actually needed, which may be indicated by a later call to e.g. \providecolors{<id-list>}.

The package also has convenient commands to define a large set of colors sharing a common prefix/suffix: \preparecolorset{HTML}{<prefix>}{<suffix>}{red,ff0000;blue,0000ff} prepares colors <prefix>red<suffix> and <prefix>blue<suffix>.

These colors could then be loaded by calling \providecolors{<prefix>red<suffix>,<prefix>blue<suffix>}.

I'm trying to implement a convenience command to avoid having to repeatedly write out the shared prefix and suffix of the colors. The syntax would be \ProvideColorsFromSet{<prefix>}{<suffix>}{<list>}.

However, the code below, which I thought was rather simple, throws errors Undefined color 'my-red' and Undefined color 'my-blue'.

I'm guessing the problem's to do with expansion, so I've tried different combinations of \exp_args:... and \exp_not:... (without really knowing what I'm doing), but to no avail. I also tried to convert the clist to a seq, and to use \..._map_function instead of \..._map_inline, but with no success.

Any help would be much appreciated. Thank you!

\documentclass{article}
\usepackage{xparse}
\usepackage{xcolor}
\definecolorsfalse
\preparecolorset{HTML}{my-}{}{red,ff0000;blue,0000ff}

\ExplSyntaxOn
\NewDocumentCommand { \ProvideColorsFromSet } { m m m } {
    \clist_set:Nn \l_tmpa_clist { #3 }
    % \exp_args:NNx \clist_set:Nn \l_tmpa_clist { #3 }
    % \clist_set:Nn \l_tmpa_clist { \exp_not:n #3 }
    \clist_map_inline:Nn \l_tmpa_clist { #1##1#2 }
    \tl_set:Nn \l_tmpa_tl { \clist_use:Nn \l_tmpa_clist {,} }
    \providecolors{ \tl_use:N \l_tmpa_tl }
    % \exp_args:NV \providecolors{ \l_tmpa_tl }
    % \providecolors{ \l_tmpa_tl }
    % \providecolors{ \exp_not:N \l_tmpa_tl }
    % \providecolors{ \exp_args:NV \exp_not:N \l_tmpa_tl } % hmm...
    % \exp_args:Nx \providecolors{ \l_tmpa_tl }
    % \exp_args:Nx \providecolors{ \tl_use:N \l_tmpa_tl }
}
\ExplSyntaxOff

\ProvideColorsFromSet{my-}{}{red,blue}

\begin{document}

\textcolor{my-red}{red}
\textcolor{my-blue}{blue}
    
\end{document}

---

Addendum:

Definition of \providecolors in xcolor:

\def\XC@preparec@lor[#1]#2[#3]#4#5%
 {\XC@sdef\@@cls{#1}\XC@edef\@@nam{#2}%
  \XC@getmodclr01{#4}{#5}{}%
   {\xdef\XC@@stack
     {\ifx\XC@@stack\@empty\else\XC@@stack,\fi{#1}{\@@nam}{#3}%
      {\ifx\@@Mod\XC@mod@ignore\@@mod\else\@@Mod\fi}{\@@clr}}}}

\let\XC@@stack\@empty

\def\XC@definecolors#1,%
 {\@ifxempty{#1}{}{\expandafter\XC@definec@lors#1=#1=:\XC@definecolors}}

\def\XC@definec@lors#1=#2=#3:%
 {\XC@edef\@@nam{#1}\XC@edef\@@arg{#2}%
  \let\next\XC@definec@l@rs\expandafter\next\XC@@stack,,\@nnil}

\def\XC@definec@l@rs#1,%
 {\ifx,#1,\let\next\remove@to@nnil\else\XC@defin@c@l@rs#1\fi\next}

\def\XC@defin@c@l@rs#1#2#3#4#5%
 {\def\@@tmp{#2}%
  \ifx\@@tmp\@@arg
    \let\xglobal@\XC@@glb\XC@@cmd[#1]{\@@nam}[#3]{#4}{#5}%
    \let\next\remove@to@nnil
  \fi}

\def\providecolors#1%
 {\let\XC@@cmd\providecolor\let\XC@@glb\xglobal@
  \expandafter\XC@definecolors#1,,}

Answer 1

You want to build a clist from the given third argument in which every item is prefixed and suffixed as indicated by the first and second argument, then pass the list to \providecolors.

\documentclass{article}
%\usepackage{xparse}% no longer needed
\usepackage{xcolor}
\definecolorsfalse
\preparecolorset{HTML}{my-}{}{red,ff0000;blue,0000ff}

\ExplSyntaxOn
\NewDocumentCommand { \ProvideColorsFromSet } { m m m }
  {
    \clist_clear:N \l_tmpa_clist
    \clist_map_inline:nn { #3 }
      {
        \clist_put_right:Nn \l_tmpa_clist { #1##1#2 }
      }
    \exp_args:NV \providecolors \l_tmpa_clist
  }
\ExplSyntaxOff

\ProvideColorsFromSet{my-}{}{red,blue}

\begin{document}

\textcolor{my-red}{red}
\textcolor{my-blue}{blue}
    
\end{document}