%%
%% The LaTeX Companion, 3ed
%%
%% Example 11-7-22 on page II-207 in "Color in formulas".
%%
%% Copyright (C) 2022 Frank Mittelbach
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3c
%% of this license or (at your option) any later version.
%%
%% See https://www.latex-project.org/lppl.txt for details.
%%

\documentclass{tlc3examargin}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{123.0pt}

  % Shorten the vertical size of the example ...
  \AtBeginDocument{%
    \setlength\abovedisplayskip{.5\abovedisplayskip}%
    \setlength\belowdisplayskip{.2\belowdisplayskip}%
    \setlength\baselineskip{8pt}%                      % <- fake for book
  }

%StartShownPreambleCommands
\usepackage{amsmath,color}
%StopShownPreambleCommands

\begin{document}
What is \textcolor{blue}{$(a+b)(a-b)$}?
\begin{align*}
  \color{blue} (a+b)^2 &= a^2 + 2ab + b^2 \\
  (a-b)^2 &=  a^2 \color{blue}- 2ab + b^2 \end{align*}
\end{document}