Tikz fit-library vs fill (overlaying nested node colors)

Question

I am trying to figure out the best way of nesting tikz nodes having a different background color. I would like the "inner" nodes to "win" concerning the background color. Currently the "outer" node ("fit") always "wins". Is there maybe a better way to aproach this?

A simple example: Nesting an inner node with background color red within an outer node with background color blue:

\documentclass{minimal}
\usepackage{tikz}
\usetikzlibrary{fit}
\begin{document}
% \begin{tikzpicture}%
%   \node (inner) [draw] {foo-inner};%
%   \node (outer) [draw,fit=(inner)] {};%
% \end{tikzpicture}%
% \begin{tikzpicture}%
%   \node (inner) [draw,fill=red] {foo-inner};%
%   \node (outer) [draw,fit=(inner)] {};%
% \end{tikzpicture}%
\begin{tikzpicture}%
  \node (inner) [draw,fill=red] {foo-inner};%
  \node (outer) [draw,fill=blue,fit=(inner)] {};%
\end{tikzpicture}%
\end{document}%

Answer 1

You can also try to send the fitting node to the background layer

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{fit,backgrounds}
\begin{document}
\begin{tikzpicture}%
  \node (inner) [draw,fill=red] {foo-inner};%
  \node (inner2) [draw,fill=green] at (2,2) {foo-inner2};%
\begin{scope}[on background layer]
  \node (outer) [draw,fill=blue,fit=(inner) (inner2)] {};%
\end{scope}
\end{tikzpicture}%
\end{document}

Answer 2

Note that this is more of hack:

If you reverse the order this "appears" (see Note below) to yield desired results:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{fit}
\begin{document}
\begin{tikzpicture}%
  \node (inner) [draw,fill=red,fill] {foo-inner};%
  \node (outer) [draw,fill=blue,fit=(inner)] {};%
\end{tikzpicture}%
\begin{tikzpicture}%
  \node (outer) [draw,fill=blue,fit=(inner)] {};%
  \node (inner) [draw,fill=red,fill] {foo-inner};%
\end{tikzpicture}%
\end{document}%

Note:

As Jake correctly points out the above solution works only because the first tikzpicture. So, just reversing the order won't work in general. But, you could repeat the node you want at the top:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{fit}
\begin{document}
\begin{tikzpicture}%
  \node (inner) [draw,fill=red,fill] {foo-inner};%
  \node (outer) [draw,fill=blue,fit=(inner)] {};%
  \node (inner) [draw,fill=red,fill] {foo-inner};%
\end{tikzpicture}%
\end{document}%

Answer 3

This is the solution from the pgfmanual.pgf:

\documentclass[border=1pt, tikz]{standalone}

\usetikzlibrary{fit, backgrounds} 

\begin{document}

\begin{tikzpicture}[level distance=8mm]
    \node (root) {root}
        child { node (a) {a} }
        child { node (b) {b}
            child { node (d) {d} }
            child { node (e) {e} } }
        child { node (c) {c} };

    \begin{pgfonlayer}{background}
        \node[fill=red!20,inner sep=0pt,circle,fit=(root) (b) (d) (e)] {};
        \node[fill=blue!20,inner sep=0pt,circle,fit=(b) (c) (e)] {};
    \end{pgfonlayer}
\end{tikzpicture}

\end{document}

If you use many nested nodes, take a look to pgfdeclarelayer and pgfsetlayers.

\pgfdeclarelayer{background}
\pgfdeclarelayer{foreground}
\pgfsetlayers{background,main,foreground}

This will define two additional layers and set their order.