Wie fülle ich Teilflächen in TikZ?

%\documentclass[border=15mm, varwidth]{standalone}
\documentclass{scrreprt}
\usepackage{selinput}
\SelectInputMappings{adieresis={ä},  germandbls={ß}}
\usepackage[ngerman]{babel}
\usepackage[T1]{fontenc} 
\usepackage{microtype}

\usepackage{tikz}
    \usetikzlibrary{shapes}
    \usetikzlibrary{intersections}

\usepackage{pgfplots}
\usepgfplotslibrary{fillbetween}
%\usepackage{mwe}
%=========
\begin{document}
%=========

\begin{tikzpicture}[
scale=0.5,
%outer sep=0pt, inner sep=0pt,
%QUADRAT/.style={},
]

% Für fillbetween
%\pgfdeclarelayer{pre main}
%\pgfsetlayers{pre main,main}
%

% Größen
\def\a{10cm}
\pgfmathsetmacro{\W}{sqrt(2)/2}

% Orientierungsfiguren
%\draw[red, fill] (0,0) circle[radius=5pt];
%\draw[brown] (0,0)--(0.5*\a,0);
%\fill[cyan] (\a,\a) circle[radius=5pt];
%\fill[] (2*\a,0.5*\a) circle[radius=5pt];

% Gesamtes Rechteck
\draw[name path=Rechteck] (0,0) rectangle (2*\a,\a);

% Figur inks
\path[name path=QuadratLINKS] (0,0) rectangle (\a,\a);
\draw[name path=KreisLINKS] (0.5*\a,0.5*\a) circle[radius=0.5*\a];

% Figur rechts
\path[name path=QuadratRECHTS] (\a,0) rectangle (2*\a,\a);
\draw[name path=KreisRECHTS] (1.5*\a,0.5*\a) circle[radius=0.5*\a];

% Diagonale
\draw[name path=DIAGONALE] (0,0) -- (2*\a,\a);

% Beschriftung
%\node[right] at (2*\a,0.5*\a) {$a$};
%\node[below] at (0.9*\a,0) {$2a$};

% Schnittpunkte links
%\fill [red,  name intersections={of=KreisLINKS and DIAGONALE}]
%(intersection-1) circle (2pt) node {1}
%(intersection-2) circle (2pt) node {2};

% Schnittpunkte rechts
\fill [red,  name intersections={of=KreisRECHTS and DIAGONALE}]
(intersection-1) circle (2pt) node {3}
(intersection-2) circle (2pt) node {4};

%% Für fillbetween
% Kreissegment OR füllen
\path [draw,blue, fill, 
name path=3and4,
intersection segments={
of=KreisRECHTS and DIAGONALE,
sequence={A1 -- B1} %[reverse]
}];

% Kreissegment OL füllen
\path [draw, orange, fill, 
name path=1and3,
intersection segments={
of=KreisLINKS and DIAGONALE,
sequence={A1 -- B1} %[reverse]
}];

%\pgfonlayer{pre main}
%\fill [
%blue!40!white,
%intersection segments={
%of=3and4 and KreisRECHTS,
%sequence={A1 -- B1[reverse]}, %
%}
%];
%\endpgfonlayer
%
\end{tikzpicture}
%=========
\end{document}
%=========