change title color

Question

I use this code to batch modify the title color of a large number of latex files

\renewcommand{\title}[1]{\renewcommand{\@title}{\color{\@titlecolor}#1}}
\newcommand{\@titlecolor}{black}
\newcommand{\titlecolor}[1]{\renewcommand{\@titlecolor}{#1}}
\makeatother

But I found that the program will get wrong results when the title has optional parameters.

\documentclass[reqno]{amsart}
\usepackage{xcolor}

\makeatletter
\renewcommand{\title}[1]{\renewcommand{\@title}{\color{\@titlecolor}#1}}
\newcommand{\@titlecolor}{black}
\newcommand{\titlecolor}[1]{\renewcommand{\@titlecolor}{#1}}
\makeatother
\begin{document}
\title[Boundary layer ansatz for the steady MHD equations]{Verification of Prandtl boundary layer ansatz for the steady electrically conducting fluids with a moving physical boundary}

\email{[email protected]}

\maketitle

\end{document}

I hope to modify this code to adapt to the situation where there are optional parameters. Every help and its description is highly welcome.

Answer 1

As so many others, don't put formatting into data holder macros, it never belongs there. Add the formatting to the macro that actually typesets the data.

Looking at the source of amsarty.cls we can deduce that within \maketitle we go through \@maketitle and end up typesetting the title via \@settitle, which is defined as

\def\@settitle{\begin{center}%
  \baselineskip14\p@\relax
    \bfseries
    \uppercasenonmath\@title
  \@title
  \end{center}%
}

thus we can patch this to add color after \bfseries therefore this works

\documentclass[reqno]{amsart}
\usepackage{xcolor,etoolbox}

\makeatletter
\newcommand{\@titlecolor}{black}
\newcommand{\titlecolor}[1]{\renewcommand{\@titlecolor}{#1}}

\patchcmd\@settitle{\bfseries}{\bfseries\color{\@titlecolor}}{}{\ERROR}


\titlecolor{orange}

\makeatother
\begin{document}

\title[Boundary layer ansatz for the steady MHD equations]{Verification of Prandtl boundary layer ansatz for the steady electrically conducting fluids with a moving physical boundary}

\email{[email protected]}

\maketitle

\end{document}