# Tikz: shade area between two bezier curves and an intersection

I'm trying to shade the area between two Bezier curves and two lines, but I can't make it work. Here is an example

\documentclass[12pts]{beamer}
\usepackage{tikz}

\begin{document}

\begin{frame}{Example}
\begin{center}
\begin{tikzpicture}

\coordinate (y) at (0,7);
\coordinate (x) at (7,0);

\draw[<->] (y) node[above] {$P$} -- (0,0) --  (x) node[right]
{$Q$};

\path[line width=0.5mm,red,draw] (0.9,0.1) .. controls (1.5,1) and (1.8,4) .. (2,6) node[above]{$S_i^{1}$};
\path[line width=0.5mm,red,draw] (0.8,0.1) .. controls (2.2,1.5) and (3.2,3.5) .. (4.7,6) node[above]{$S_i^{2}$};
\path[line width=0.5mm,blue,draw] (0.85,0.1) .. controls (2,2) and (3,4) .. (3.7,6) node[above]{$S^{\text{Data}}$};
%  \addplot[gray!50] fill between[of=A and B];
\draw[black,dashed] (0,1.5) node[left]{$P_0$} -- (5.6,1.5);
\draw[black,dashed] (0,5) node[left]{$P_1$} -- (5.6,5);
\end{tikzpicture}
\end{center}
\end{frame}
\end{document}

The output is three Bezier curves, two red and one blue, plus two horizontal lines. I want to shade the areas between each red curve and the blue one, between the lines defined by P_1 and P_2. .

Thanks.

If you were using pgfplots it'd be easy with the fillbetween library, which can also work on tikzpicture with some changes, but you can also just use a clip in this case for the dashed lines and then use the colored lines as contour.

I also added the backgrounds library to place the filling behind the colored lines.

## Code

\documentclass[12pts]{beamer}
\usepackage{tikz}

\usetikzlibrary{backgrounds}

\begin{document}
\begin{frame}{Example}
\begin{center}
\begin{tikzpicture}
\coordinate (y) at (0,7);
\coordinate (x) at (7,0);

\draw[<->] (y) node[above] {$P$} -- (0,0) --  (x) node[right]
{$Q$};

\draw[line width=0.5mm,red] (0.9,0.1) .. controls (1.5,1) and (1.8,4) .. (2,6) node[above]{$S_i^{1}$};
\draw[line width=0.5mm,red] (0.8,0.1) .. controls (2.2,1.5) and (3.2,3.5) .. (4.7,6) node[above]{$S_i^{2}$};
\draw[line width=0.5mm,blue] (0.85,0.1) .. controls (2,2) and (3,4) .. (3.7,6) node[above]{$S^{\text{Data}}$};

\draw[black,dashed] (0,1.5) node[left]{$P_0$} -- (5.6,1.5);
\draw[black,dashed] (0,5) node[left]{$P_1$} -- (5.6,5);

\begin{scope}[on background layer]
\clip (1,1.5) rectangle (5.6,5);
\fill[green, opacity=.5] (0.9,0.1) .. controls (1.5,1) and (1.8,4) .. (2,6) -- (3.7,6) .. controls (3,4) and (2,2) .. (0.85,0.1) -- cycle;
\fill[red, opacity=.5] (4.7,6) ..controls (3.2,3.5) and (2.2,1.5) .. (0.8,0.1) -- (0.85,0.1) .. controls (2,2) and (3,4) .. (3.7,6) -- cycle;
\end{scope}
\end{tikzpicture}
\end{center}
\end{frame}
\end{document}