Access to color by color series name does not work

[schtandard]

(I reported this bug via email a couple years ago, but it seems maintenance has changed, so I report it again here.)

The following MWE produces a black square even though it should produce a blue one according to the documentation.

\documentclass{article}

\usepackage{xcolor}

\begin{document}

\definecolorseries{foo}{rgb}{last}{blue}{red}
\resetcolorseries{foo}

\textcolor{foo}{\rule{1ex}{1ex}}

\end{document}

From what I understood, this is due to the fact that the second argument of \xcolor@ in \\color@foo is empty.

As a workaround, one can use the (undocumented) syntax \color{foo!!}, but the documented \color{foo} would be preferred.

[u-fischer]

I'm not sure it is a bug. The code quite explicitly stores the color without backend, and nearly all examples use the !!+ prefix.

But the following should work and set the color with backend:

\documentclass{article}

\usepackage{xcolor}

\makeatletter
\def\XC@resetcolorseries[#1]#2%
 {\begingroup
  \@namexdef
   {\@backslashchar color@#2}{\noexpand\xcolor@{}{}\@nameuse{\@backslashchar colorB@#2}}%
  \xglobal\colorlet{foo}{foo!}% <----
  \XC@let@Nc\@@tmp{\@backslashchar colorD@#2}%
  \edef\@@met{\expandafter\@firstoftwo\@@tmp}%
  \edef\@@tmp{\expandafter\@secondoftwo\@@tmp}%
  \ifx\@@met\XC@met@step\else
    \edef\@@scl{\@ifxempty{#1}\colorseriescycle{#1}}%
    \expandafter\XC@calc@\@@tmp,,,,:D%
  \fi
  \@namexdef{\@backslashchar colorS@#2}{\@@tmp}%
  \endgroup}
\makeatother

\begin{document}

\definecolorseries{foo}{rgb}{last}{blue}{red}
\resetcolorseries[4]{foo}
  
xx \textcolor{foo}{\rule{1ex}{1ex}}

\end{document}

[schtandard]

I think the documentation is pretty explicit that just using the color series name should be fine, e.g.

A ⟨name⟩ denotes the declared name (or the name to be declared) of a color or a color series; [...]

from section 2.3.1 or

[...] commands like \color{foo} or \color{foo!!+} may be used without restrictions [...]

from section 2.9.4.

(Uwe Kern also seemed to agree: He wrote "das wirkt tatsächlich wie ein Bug" in reply to my email reporting this.)

[schtandard]

Regarding your suggestion: It seems to only work as long as the color series isn't stepped:

\documentclass{article}

\usepackage[T1]{fontenc}
\usepackage{lmodern}

\usepackage{xcolor}

\makeatletter
\def\XC@resetcolorseries[#1]#2%
 {\begingroup
  \@namexdef
   {\@backslashchar color@#2}{\noexpand\xcolor@{}{}\@nameuse{\@backslashchar colorB@#2}}%
  \xglobal\colorlet{foo}{foo!}% <----
  \XC@let@Nc\@@tmp{\@backslashchar colorD@#2}%
  \edef\@@met{\expandafter\@firstoftwo\@@tmp}%
  \edef\@@tmp{\expandafter\@secondoftwo\@@tmp}%
  \ifx\@@met\XC@met@step\else
    \edef\@@scl{\@ifxempty{#1}\colorseriescycle{#1}}%
    \expandafter\XC@calc@\@@tmp,,,,:D%
  \fi
  \@namexdef{\@backslashchar colorS@#2}{\@@tmp}%
  \endgroup}
\makeatother

\begin{document}

\definecolorseries{foo}{rgb}{last}{blue}{red}
\resetcolorseries{foo}

% works now
\textcolor{foo}{\rule{1ex}{1ex}}

% always worked
\textcolor{foo!!+}{\rule{1ex}{1ex}}

% still doesn't work
\textcolor{foo}{\rule{1ex}{1ex}}

\end{document}

[u-fischer]

I think the documentation is pretty explicit that just using the color series name should be fine,

I didn't find it very explicit. On the whole I had the impression that it doesn't really consider the case of using the color without stepping. In any case: one must set the color also in the step code. Be aware that foo always refers to the currently active color of a series.

\documentclass{article}

\usepackage{xcolor}

\makeatletter
\def\XC@resetcolorseries[#1]#2%
 {\begingroup
  \@namexdef
   {\@backslashchar color@#2}{\noexpand\xcolor@{}{}\@nameuse{\@backslashchar colorB@#2}}%
  \xglobal\colorlet{foo}{foo!}% <----
  \XC@let@Nc\@@tmp{\@backslashchar colorD@#2}%
  \edef\@@met{\expandafter\@firstoftwo\@@tmp}%
  \edef\@@tmp{\expandafter\@secondoftwo\@@tmp}%
  \ifx\@@met\XC@met@step\else
    \edef\@@scl{\@ifxempty{#1}\colorseriescycle{#1}}%
    \expandafter\XC@calc@\@@tmp,,,,:D%
  \fi
  \@namexdef{\@backslashchar colorS@#2}{\@@tmp}%
  \endgroup}
  
\def\XC@step#1%
 {\let\xcolor@\@gobbletwo\edef\@@tmp{\@nameuse{\@backslashchar color@#1}}%
  \expandafter\XC@st@p\@@tmp{#1}%
  \@namexdef{\@backslashchar color@#1}{\noexpand\xcolor@{}{}{\@@mod}{\@@tmp}}%
  \xglobal\colorlet{foo}{foo!}% <----
  }  
\makeatother

\begin{document}

\definecolorseries{foo}{rgb}{last}{blue}{red}
\resetcolorseries[2]{foo}

% works now
\textcolor{foo}{\rule{1ex}{1ex}}

\expandafter\show\csname \string\color@foo\endcsname
% always worked
\textcolor{foo!!+}{\rule{1ex}{1ex}}

\expandafter\show\csname \string\color@foo\endcsname
% still doesn't work
\textcolor{foo}{\rule{1ex}{1ex}}

\end{document}

[schtandard]

I didn't find it very explicit. On the whole I had the impression that it doesn't really consider the case of using the color without stepping.

Hmm, ok. I thought stating that one may use \color{foo} after saying \definecolorseries{foo}{rgb}{last}{red}{blue}\resetcolorseries[10]{foo} was conclusive. And section 2.3.1 / table 4 do state that the ⟨postfix⟩ in an ⟨expr⟩ may be empty. But anyway, even if this is not considered a bug, providing this syntax seems like an improvement to the package to me, so I hope it can be included in the next version.

Your last MWE looks good to me. At least I couldn't find any issues playing around with it for a bit.

Thank you!