change the appearance of the equation by several levels

Question

I use this code to change the appearance of the equation, but it is full of errors even if at the end it gives a result, please help me to correct it. Thank you .

\documentclass{article}
\usepackage{empheq}
\usepackage{xcolor}

\newcommand{\coloredeq}[1]{
  \begin{empheq}[box=\colorbox{red}]{align*}
    #1
  \end{empheq}
}

\let\oldeqnarray\eqnarray*
\let\endoldeqnarray\endeqnarray*

\renewenvironment{eqnarray*}{
  \begin{empheq}[box=\colorbox{red}]{align*}
}{
  \end{empheq}
}

\begin{document}
\begin{eqnarray*}
  a &= b \\
  c &= d
\end{eqnarray*}

\end{document}

Answer 1

The old way to copy a starred environment would've been

\expandafter\let\expandafter\oldeqnarray\csname eqnarray*\endcsname

but nowadays we have \NewEnvironmentCopy:

\NewEnvironmentCopy{oldeqnarray}{eqnarray*}

However, given that there is no real reason to use {eqnarray} you can safely lose the copy. A better way to define wrappers around {empheq} shown in Sec. 6 of the documentation.

\documentclass[twocolumn]{article}

\usepackage{empheq}
\usepackage{xcolor}

% if you must
\NewEnvironmentCopy{oldeqnarray}{eqnarray*}

% one possibility
\renewenvironment{eqnarray*}{%
   \empheq[box=\colorbox{red}]{align*}%
  }{%
   \endempheq
}

% a better one
\newenvironment{foo}[1][red]{%
   \setkeys{EmphEqEnv}{align*}%
   \setkeys{EmphEqOpt}{box=\colorbox{#1}}%
   \EmphEqMainEnv
  }{%
   \endEmphEqMainEnv
}

\begin{document}
See
\begin{oldeqnarray}
please & don't & use\ me
\end{oldeqnarray}
or
\begin{eqnarray*}
  a &= b \\
  c &= d
\end{eqnarray*}
but I'd prefer
\begin{foo}
a&=b\\
c&=d
\end{foo}
and
\begin{foo}[blue!10]
a&=b\\
c&=d
\end{foo}

\end{document}

Answer 2

I'm not sure why using eqnarray* as the environment's name. If you really want to save the meaning of eqnarray*, do

\NewEnvironmentCopy{oldeqnarray}{eqnarray*}

but I see no reason to, because eqnarray is buggy under many respects.

\documentclass{article}
\usepackage{empheq}
\usepackage{xcolor}

\NewDocumentEnvironment{boxedalign*}{b}{%
  \setlength{\fboxsep}{2\arraycolsep}%
  \begin{empheq}[box=\colorbox{red!40}]{align*}
    #1
  \end{empheq}%
}{\ignorespacesafterend}

\begin{document}

Some text before the display
some text before the display
some text before the display
\begin{boxedalign*}
  a &= b \\
  c &= d+k+m+n
\end{boxedalign*}
Some text after the display
some text after the display
some text after the display

\end{document}

With the b argument type, you tell LaTeX to absorb the environment's content and pass it as #1.

I also suggest a bit more separation between the edges of the box and the contents.