I am constructing an image which consists of multiple tikzpictures (because some parts of them otherwise would interfere with other parts). I would like to place a background image behind all of them consisting of multiple nodes connecting the nodes on the main layer. With remember picture I already can adress nodes defined in the other tikzpicture. With overlay I managed to place the tikzpicture with the background layer on top of the other tikzpicture(s). I suspect the problem hast to do with using 'overlay'. Is there something like 'underlay'?
This is a (extremely simplified) MWE to show what I mean:
\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{positioning, shapes}
\usepackage{xcolor}
\pgfdeclarelayer{background}
\pgfdeclarelayer{foreground}
\pgfsetlayers{background,main,foreground}
\begin{document}
\begin{tikzpicture}[remember picture]
\node[circle, fill=blue] (circle) {Main};
\node[circle, fill=blue, right=of circle] (circle2) {Main2};
\node[circle, fill=blue, below right=of circle] (circle3) {Main3};
\end{tikzpicture}
\begin{tikzpicture}[remember picture, overlay]
\begin{pgfonlayer}{background}
\node[circle, fill=green, minimum height=1cm, below right=-5pt and -5pt of circle] (circle-bg) {BG};
\end{pgfonlayer}
\node[circle, fill=yellow, minimum height=1cm, below right=-5pt and -5pt of circle-bg] (circle-main) {overlay};
\end{tikzpicture}
\end{document}
I would like the green circle also be behind the blue circle.