\begin{tikzpicture}[scale=2]
\filldraw[cyan] (1.06,1.06)--(0,0)--(-1.06,1.06)--(-1.05,1.15) .. controls (-.2,.3) and (.2,.3) .. (1.05,1.15);
\filldraw[lime] (1.06,-1.06)--(0,0)--(-1.06,-1.06)--(-1.05,-1.15) .. controls (-.2,-.3) and (.2,-.3) .. (1.05,-1.15);
\draw (-1.05,1.15) .. controls (-.2,.3) and (.2,.3) .. (1.05,1.15);
\draw (-1.05,-1.15) .. controls (-.2,-.3) and (.2,-.3) .. (1.05,-1.15);
\draw[->] (-1.5,0) -- (1.5,0) node[right] {$\xi$};
\draw[->] (0,-1.5) -- (0,1.5) node[above] {$\eta$};
\draw[very thin, red, dashed] (-1.06,-1.06) -- (1.06,1.06);
\draw[very thin, red, dashed] (-1.06,1.06) -- (1.06,-1.06);
\end{tikzpicture}