I made a logo for the math software Maxima with Tikz, using the contour package and transparency, but the contour seems to show up "behind" the letters in addition to around them. This shows up when using transparency. How to avoid the contour behind letters?
\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{positioning}
\usepackage[outline]{contour}
\begin{document}
\scalebox{20} {
\begin{tikzpicture}
\draw node [rectangle,
rounded corners=2.55mm,
draw=white,
very thin,
fill=blue!10!white,
inner sep=2.55mm,
anchor=north east] (logo) {};
\contourlength{0.05mm};
\node (xi) [ left=-3.7mm of logo] {
\large\bf\color{blue!20!white}\contour{white}{$\zeta$}};
\contourlength{0.05mm};
\node (sigma) [right=-4.7mm of logo] {
\large\bf\color{blue!20!white}\contour{white}{$\Sigma$}};
\contourlength{0.15mm};
\node (M) [opacity=.6] at (logo) {
\bf\color{red!50!white}\contour{white}{M}};
\end{tikzpicture}
}
\end{document}