Change only the colour of the (bracketed part) of the title of a theorem

Question

How may I change only the colour of the title of a theorem, i.e. the part that appears in brackets after say, "Theorem", "Definition" etc. when using the amsthm package?

I have tried passing \color{red} to the Theorem Head Font (see below) but this also changes the first part, that is not in brackets.

\newtheoremstyle{note}% (name)
    {}% (space above)
    {}% (space below)
    {}% (body font)
    {}% (indent amount)
    {\bfseries\color{red}}% (Theorem head font)
    {:}% (Punctuation after theorem head)
    {.5em}% (Space after theorem head)
    {}% (Theorem head spec (can be left empty, meaning ‘normal’))

To clarify, when saying

\begin{definition}[foo] an explanation \end{definition}

I would like only "foo" to be red but definition to remain as i had defined it in the theorem head font.

Answer 1

You could use the last argument of \newtheoremstyle for this. The arguments of \thmname, \thmnumber and \thmnote are only printed if the relevant field is present.

\documentclass{article}

\usepackage{amsthm}
\usepackage{xcolor}

\newtheoremstyle{note}% (name)
    {}% (space above)
    {}% (space below)
    {}% (body font)
    {}% (indent amount)
    {\bfseries}% (Theorem head font)
    {:}% (Punctuation after theorem head)
    {.5em}% (Space after theorem head)
    {\thmname{#1}\thmnumber{ #2}\thmnote{ (\textcolor{red}{#3})}}
    % ^^(Theorem head spec (can be left empty, meaning ‘normal’))

\theoremstyle{note}
\newtheorem{theorem}{Theorem}

\begin{document}

\begin{theorem}[Named theorem]
    This theorem has a name.
\end{theorem}

\begin{theorem}
    This theorem does not.
\end{theorem}

\end{document}