Farbverlauf Legende TikZ

\documentclass{standalone}
\usepackage{pgfplots}

\pgfplotsset{compat=1.14}

\begin{document}

\begin{tikzpicture}
\begin{axis}[%
domain=-14:0,
xmin=-14,
xmax=0,
ymin=2.5,
ymax=4.2,
scaled ticks=false,
legend style={at={(1,0)},anchor=south east},
legend entries={100\%, 90\%, 70\%, 50\%, 30\%, 10\%},
legend image code/.code={
    \fill [#1] (0cm,-0.1cm) rectangle (0.6cm,0.1cm);
},
cycle list={[samples of colormap={6 of viridis}]},
]
\addplot {0.0140*x + 4.18};
\addplot {0.0142*x + 4.01};
\addplot {0.0146*x + 3.9};
\addplot {0.0144*x + 3.7};
\addplot {0.014*x + 3.6};
\addplot {0.015*x + 3.4};
\end{axis}
\end{tikzpicture}

\end{document}

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{tikz}
\usepackage{pgfplots}
\pgfplotsset{compat=1.14}
\usepackage{siunitx}
\begin{document}

\begin{figure}
  \centering 
  \begin{tikzpicture}
    \begin{axis}[%
      domain=-14:0,
      scale=0.85,
      width=0.95\linewidth,
      height=9cm,
      scale only axis,
      separate axis lines,
      every outer x axis line/.append style={black},
      every x tick label/.append style={font=\color{black}\footnotesize},
      every y tick label/.append style={font=\color{black}\footnotesize},
      xmin=-14,
      xmax=0,
      ymin=2.5,
      ymax=4.2,
      xlabel={Strom in $\SI{}{\ampere}$},
      ylabel={Spannung in $\SI{}{\volt}$},
      ylabel near ticks,
      xlabel near ticks,
      axis background/.style={fill=white},
      scaled ticks=false,
      y tick label style={/pgf/number format/.cd,use comma,fixed,precision=3},
      legend style={at={(1,0)},anchor=south east},
      every axis plot/.style = {
        no markers, very thick, solid,
        legend image code/.code={
          \filldraw[#1] (0cm,-0.1cm) rectangle (0.6cm,0.1cm);
        },
      }
      ]
      \addplot[red!0!blue] {0.0140*x + 4.18};
      \addlegendentry{$\SI{100}{\percent}$}
      \addplot[red!20!blue] {0.0142*x + 4.01};
      \addlegendentry{$\SI{90}{\percent}$}
      \addplot[red!40!blue] {0.0146*x + 3.9};
      \addlegendentry{$\SI{70}{\percent}$}
      \addplot[red!60!blue] {0.0144*x + 3.7};
      \addlegendentry{$\SI{50}{\percent}$}
      \addplot[red!80!blue] {0.014*x + 3.6};
      \addlegendentry{$\SI{30}{\percent}$}
      \addplot[red!100!blue] {0.015*x + 3.4};
      \addlegendentry{$\SI{10}{\percent}$}
    \end{axis}
  \end{tikzpicture}
  \label{fig:Test}
\end{figure} 
\end{document}

\documentclass[margin=5pt]{standalone}
\usepackage[utf8]{inputenc}
\usepackage{pgfplots}% lädt auch tikz
\pgfplotsset{compat=1.14}

% Farbserie mit 100 Schritten definieren
\definecolorseries{plotcolor}{rgb}{last}{yellow}{blue}
\resetcolorseries[100]{plotcolor}

% Eigener Legendenstil
\pgfplotsset{
  my area legend/.style={
    draw=none,
    inner ysep=0pt,
    inner xsep=3pt,
    row sep=-.5\pgflinewidth,
    nodes={inner ysep=0pt},
    legend image code/.code={%
      \filldraw[#1,draw=none] (0cm,-0.08cm) rectangle (0.4cm,0.08cm);
    }
  },
  myplotcolor/.style={plotcolor!![#1]}
}

\usepackage{siunitx}
\begin{document}
\begin{tikzpicture}
  \begin{axis}[%
    domain=-14:0,
    scale=0.85,
    width=0.95\linewidth,
    height=9cm,
    scale only axis,
    separate axis lines,
    every outer x axis line/.append style={black},
    every x tick label/.append style={font=\color{black}\footnotesize},
    every y tick label/.append style={font=\color{black}\footnotesize},
    xmin=-14,
    xmax=0,
    ymin=2.5,
    ymax=4.2,
    xlabel={Strom in $\si{\ampere}$},
    ylabel={Spannung in $\si{\volt}$},
    axis background/.style={fill=white},
    scaled ticks=false,
    y tick label style={/pgf/number format/.cd,use comma,fixed,precision=3},
    legend style={
      at={(1,0)},yshift=5pt,xshift=-1em,anchor=south east,
      my area legend,
      name=legend
    },
    every axis plot/.style={no markers,very thick,solid}
  ]
    % Legende
    \pgfplotsinvokeforeach {100,90,...,10}{
      \addlegendimage{myplotcolor=#1,my area legend}
      \addlegendentry{}
    }
    %Plots
    \addplot[myplotcolor=100] {0.0140*x + 4.18};
    \addplot[myplotcolor=90] {0.0142*x + 4.01};
    \addplot[myplotcolor=70] {0.0146*x + 3.9};
    \addplot[myplotcolor=50] {0.0144*x + 3.7};
    \addplot[myplotcolor=30] {0.014*x + 3.6};
    \addplot[myplotcolor=10] {0.015*x + 3.4};
  \end{axis}
  % Beschriftung der Legende
  \path[nodes={font=\tiny,inner xsep=0pt}]
    (legend.north west) node [anchor= east] {$100$}
    (legend.west) node [anchor= east] {$50$}
    (legend.south west) node [anchor= east] {$0$}
    (legend.north) node [anchor=south] {Par\,[$\si{\percent}$]}
  ;
\end{tikzpicture}
\end{document}