I'm trying to shade an area with tree colors and this works as expected. But I want the filling to be half-transparent so I included opacity=0.5 in my code and it seems that LaTex adds some gray into the picture.
Does anybody know why this is happening and how I can avoid this gray layer (i.e. the shading should be half-transparent but with no gray)?
\documentclass[11pt, a4paper]{scrartcl}
\usepackage{tikz}
\usepackage{pgfplots}
\pgfplotsset{compat=1.12}
\usepgfplotslibrary{fillbetween}
\usepgflibrary{shadings}
\begin{document}
\begin{tikzpicture}[scale=1,>=latex,font= \footnotesize]
\begin{axis}[axis lines = middle,grid=both,ymin=-3.1, ymax=3.4]
\draw[red, dashed,name path= C] (8,3) -- (8,-3);
\addplot[thick,blue!80!black,domain=-sqrt(8):sqrt(8),name path=A] plot (\x*\x,\x) ;
\addplot[fill opacity=1,top color=blue,bottom color=blue,middle color=white] fill between[of=A and C];
\end{axis}
\end{tikzpicture}
\begin{tikzpicture}[scale=1,>=latex,font= \footnotesize]
\begin{axis}[axis lines = middle,grid=both,ymin=-3.1, ymax=3.4]
\draw[red, dashed,name path= C] (8,3) -- (8,-3);
\addplot[thick,blue!80!black,domain=-sqrt(8):sqrt(8),name path=A] plot (\x*\x,\x) ;
\addplot[fill opacity=0.5,top color=blue,bottom color=blue,middle color=white] fill between[of=A and C];
\end{axis}
\end{tikzpicture}
\end{document}