Different color inside the proof environment, only when I want

Question

There is the proof environment in the amsmath package. If I want to globally set the text color inside the proof environment, I can use the command

\AtBeginEnvironment{proof}{\color{blue}}

as illustrated in Change Color of Proof.

However, suppose that I want to change the color inside the proof only sometimes. There may be two (or more) options to achieve this.

Option 1: Define two environments proof and blueproof, where proof is the usual environment and blueproof is a custom-made environment that is a copy of the proof except that the color inside the proof is blue.

Option 2: Put appropriate options inside the proof, for example,

\begin{proof}[here my options to change the color]
...
\end{proof}

Either option is fine for me. How can I make the proof color blue when necessary?

Edit: Sadly, the below answer does not work.

Running the provided code, I instead obtain:

with error messages that Undefined control sequence. \begin{Proof}[blue], etc.

amsmath doesn't define any proof environment;, but amsthm does. But you haven't said what theorem package you are using, if any. So this question can't be answered without more information. Please supply a small compilable example, beginning with \documentclass and ending with \end{document}, that shows what you've tried. — barbara beeton, Aug 30 '22 at 15:49

egreg · Accepted Answer · 2022-08-30T16:03:39.423

I suggest going to a new environment, because the optional argument is already reserved for an alternative header.

\documentclass{article}
\usepackage{amsthm}
\ExplSyntaxOn
\NewDocumentEnvironment{Proof}{O{}}
 {
  \color_group_begin:
  \keys_set:nn { eigenvalue/proof } { #1 }
  \tl_if_empty:VF \l__eigenvalue_proof_color_tl
   {
    \color_select:V \l__eigenvalue_proof_color_tl
   }
  \tl_if_empty:VTF \l__eigenvalue_proof_name_tl
   {
    \proof
   }
   {
    \proof[\l__eigenvalue_proof_name_tl]
   }
 }
 {
  \color_group_end:
  \endproof
 }
\keys_define:nn { eigenvalue/proof }
 {
  color .tl_set:N = \l__eigenvalue_proof_color_tl,
  name  .tl_set:N = \l__eigenvalue_proof_name_tl,
 }
\cs_generate_variant:Nn \color_select:n { V }
\ExplSyntaxOff
\begin{document}
\begin{Proof}
This is a proof.
\end{Proof}
\begin{Proof}[name=Proof of the main theorem]
This is a proof.
\end{Proof}
\begin{Proof}[color=blue!70]
This is a proof.
\end{Proof}
\begin{Proof}[name=Proof of the main theorem,color=red!60!blue]
This is a proof.
\end{Proof}
\end{document}

You may also define shorter keys, for instance blue to stand for color=blue!70 (it's best to never use pure colors).

\documentclass{article}
\usepackage{amsthm}
\ExplSyntaxOn
\NewDocumentEnvironment{Proof}{O{}}
 {
  \color_group_begin:
  \keys_set:nn { eigenvalue/proof } { #1 }
  \tl_if_empty:VF \l__eigenvalue_proof_color_tl
   {
    \color_select:V \l__eigenvalue_proof_color_tl
   }
  \tl_if_empty:VTF \l__eigenvalue_proof_name_tl
   {
    \proof
   }
   {
    \proof[\l__eigenvalue_proof_name_tl]
   }
 }
 {
  \color_group_end:
  \endproof
 }
\keys_define:nn { eigenvalue/proof }
 {
  color .tl_set:N = \l__eigenvalue_proof_color_tl,
  name  .tl_set:N = \l__eigenvalue_proof_name_tl,
  blue .meta:n = { color = blue!70 },
 }
\cs_generate_variant:Nn \color_select:n { V }
\ExplSyntaxOff
\begin{document}
\begin{Proof}
This is a proof.
\end{Proof}
\begin{Proof}[name=Proof of the main theorem]
This is a proof.
\end{Proof}
\begin{Proof}[color=blue!70]
This is a proof.
\end{Proof}
\begin{Proof}[blue]
This is a proof.
\end{Proof}
\begin{Proof}[name=Proof of the main theorem,color=red!60!blue]
This is a proof.
\end{Proof}
\end{document}

If you want to color also the tombstone

\documentclass{article}
\usepackage{amsthm}
\ExplSyntaxOn
\NewDocumentEnvironment{Proof}{O{}}
 {
  \keys_set:nn { eigenvalue/proof } { #1 }
  \tl_if_empty:VF \l__eigenvalue_proof_color_tl
   {
    \cs_set:Npx \qedsymbol
     {
      \color_select:V \exp_not:N \l__eigenvalue_proof_color_tl \exp_not:o { \qedsymbol }
     }
   }
  \color_group_begin:
  \tl_if_empty:VF \l__eigenvalue_proof_color_tl
   {
    \color_select:V \l__eigenvalue_proof_color_tl
   }
  \tl_if_empty:VTF \l__eigenvalue_proof_name_tl
   {
    \proof
   }
   {
    \proof[\l__eigenvalue_proof_name_tl]
   }
 }
 {
  \color_group_end:
  \endproof
 }
\keys_define:nn { eigenvalue/proof }
 {
  color .tl_set:N = \l__eigenvalue_proof_color_tl,
  name  .tl_set:N = \l__eigenvalue_proof_name_tl,
  blue .meta:n = { color = blue!70 },
 }
\cs_generate_variant:Nn \color_select:n { V }
\ExplSyntaxOff
\begin{document}
\begin{Proof}
This is a proof.
\end{Proof}
\begin{Proof}[name=Proof of the main theorem]
This is a proof.
\end{Proof}
\begin{Proof}[color=blue!70]
This is a proof.
\end{Proof}
\begin{Proof}[blue]
This is a proof.
\end{Proof}
\begin{Proof}[name=Proof of the main theorem,color=red!60!blue]
This is a proof.
\end{Proof}
\end{document}

Thanks a lot for your answer, but sadly your code does not work on my environment (see my edited post). Is there any requirement for your code to run? — Laplacian, Sep 01 '22 at 00:50
@eigenvalue Update your TeX system. Please, show also the error messages you get. — egreg, Sep 01 '22 at 08:16

Different color inside the proof environment, only when I want

1 Answers1