LaTeX command for equal (=)

Question

Is there a LaTeX command for the equal (=) sign? I'd like to overwrite the command so that all equal signs in my document are colored green. But it seems I cannot do it with just the symbol =, which is not a command.

Answer 1

Ten years later, a better implementation.

\documentclass{article}
\usepackage{xcolor}
% preserve the original equals
% and make = math active
\AtBeginDocument{%
  \mathchardef\standardequals=\mathcode= \mathcode="8000
}
% define math active = to be green \standardequals
\ExplSyntaxOn
\cs_new_protected:Nn \mika_greenequals: { \mathcolor{green!70!red}{\standardequals} }
\char_set_active_eq:NN = \mika_greenequals:
\ExplSyntaxOff
\begin{document}
$a\standardequals b$
$a=b$
\end{document}

The top line is to show that spacing is right.

original answer

Every character can be turned into a command in math mode, but some special tricks are needed.

\documentclass{article}
\usepackage{xcolor}
\begingroup\lccode~==
\lowercase{\endgroup\def~}{\mathrel{\textcolor{green}{\standardequals}}}
\edef\standardequals{\mathchar\the\mathcode`=\relax}
\AtBeginDocument{\mathcode`=\string"8000 }
\begin{document}
$a=b$
\end{document}