%% 
%%  The LaTeX Graphics Companion, 2ed (first printing May 2007)
%% 
%%  Example 11-2-16 on page 731.
%% 
%%  Copyright (C) 2007 Michel Goossens, Frank Mittelbach, Denis Roegel, Sebastian Rahtz, Herbert Vo\ss 
%% 
%%  It may be distributed and/or modified under the conditions
%%  of the LaTeX Project Public License, either version 1.3
%%  of this license or (at your option) any later version.
%% 
%%  See http://www.latex-project.org/lppl.txt for details.
%% 

\documentclass{ttctexagray}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{111.83385pt}
\setlength\parindent{0pt}
\StartShownPreambleCommands
\usepackage{xcolor}
\colorlet{test}{yellow!90} \colorlet{anti}{-test}
\StopShownPreambleCommands
\begin{document}
\newcommand\blob[1]{\fbox{\color{#1}\rule{1.5cm}{5mm}}}
\blob{test}                \blob{anti}         \\
\blob{test!50!anti}        \blob{gray}         \\
\blob{rgb,1:test,1;anti,1}
\end{document}