I need to create a plot with an area filled outside a circle. Because the filled area has to be colored with 2 different colors, simply applying the odd-even rule did not work. This, it worked for one color or the other but not for both. That is not my problem. It is solved.
From other posts in this forum I was pointed to this article: http://www.texample.net/tikz/examples/venn-diagram/. The applicable example is the yellow circle A with the area of circle B cut out.
I modified the example slightly to make visible what I did. This is the MWE code:
\documentclass[a4]{article}
\usepackage{tikz}
\begin{document}
\def\firstcircle{(0,0) circle (1.5cm)}
\def\secondcircle{(45:2cm) circle (1.5cm)}
\begin{tikzpicture}
\begin{scope}[shift={(6cm,-10cm)}]
\draw[help lines] (0,0) grid (4,5);
\begin{scope}[even odd rule]% first circle without the second
\draw[clip] \secondcircle (-3,-3) rectangle (3,3);
\fill[yellow] \firstcircle;
\end{scope}
\draw \firstcircle node {$A$};
\draw \secondcircle node {$B$};
\end{scope}
\end{tikzpicture}
\end{document}
This works and solved my own problem as well after modifying the code so it suited my picture.
venn-diagram in square http://www.linxtech.net/~jlinkels/links/venn-diagram.jpg
The nagging question is, why does it work? What makes it happen that the yellow circle is colored, but the area occupied by circle B is empty. I understand that B remains empty when the square is filled. But first the square is not filled, does it still apply that B remains empty? Adding circle A is completely opaque (no pun intended). Why would A fill, but still leave B's area empty?