Legend entry for each point in scatter plot with matching color

Question

I have a file with multiple columns, of which I create scatter plots where positions are given from column 1 vs column 2, while the mark color is based on column 3. I now wish to automatically create a legend with an entry for each color.

The following solution works for a limited number of colors (6-7). It uses a colormap for the dots in the scatter plot and a color cycle list, with equal colors as in the colormap, for the legend images.

Question: How do I generalize it to use with more dots than colors? This would require to use the colormap to draw the legend images instead of the cycle list. If this was possible, one could also use predefined colormaps, instead of defining everything anew.

I think the problem is, that the \addlegendentry in the loop is unable to retain the color of the current scattered mark, otherwise the solution would be a lot easier.

\documentclass{standalone}
\usepackage{pgfplots}
\usepackage{pgfplotstable}
\usepackage{filecontents}
\pgfplotsset{compat=newest}

\usetikzlibrary{pgfplots.groupplots}


\begin{filecontents}{data.out}
0 1  2
1 3  4
2 5  6
3 7  8
4 11 12
5 13 14
\end{filecontents}


\pgfplotstableread{data.out}\datatable

\pgfplotsset{
    select row/.style={
        x filter/.code={\ifnum\coordindex=#1\else\def\pgfmathresult{}\fi}
    }
}


\definecolor{RYB1}{RGB}{141, 211, 199}
\definecolor{RYB2}{RGB}{255, 255, 179}
\definecolor{RYB3}{RGB}{190, 186, 218}
\definecolor{RYB4}{RGB}{251, 128, 114}
\definecolor{RYB5}{RGB}{128, 177, 211}
\definecolor{RYB6}{RGB}{253, 180, 98}
\definecolor{RYB7}{RGB}{179, 222, 105}

\pgfplotscreateplotcyclelist{colorbrewer-RYB}{
{RYB1!50!black,fill=RYB1},
{RYB2!50!black,fill=RYB2},
{RYB3!50!black,fill=RYB3},
{RYB4!50!black,fill=RYB4},
{RYB5!50!black,fill=RYB5},
{RYB6!50!black,fill=RYB6},
%{RYB7!50!black,fill=RYB7},
}

\pgfplotsset{colormap={MyRYB}{
    rgb255=(141, 211, 199)
    rgb255=(255, 255, 179)
    rgb255=(190, 186, 218)
    rgb255=(251, 128, 114)
    rgb255=(128, 177, 211)
    rgb255=(253, 180, 98)
%    rgb255=(179, 222, 105)
  }
}


\begin{document}

\begin{tikzpicture}
  \begin{groupplot}[
    group style={
      group name=myplot,
      group size=2 by 1,
      horizontal sep=60pt,
    },
    legend pos=outer north east,
    % colormap/bluered,
    colormap name={MyRYB},
    scatter/use mapped color=
    {draw=black,fill=mapped color},
    cycle list name=colorbrewer-RYB,
    ]

    \nextgroupplot[
    legend to name=grouplegend,
    legend columns=-1,
    legend style={draw=none, /tikz/every even column/.append style={column sep=5pt}},
    legend image code/.code={%
      \draw[#1] (0cm,-0.1cm) rectangle (0.3cm,0.1cm);
    },
    ]


    \pgfplotsinvokeforeach  {0,...,6}{
    \addplot+[scatter,scatter src=\thisrowno{2},thick]
    table [ x index={0}, select row=#1, y index={1}] {data.out};
    \addlegendentry {\pgfplotstablegetelem{#1}{2}\of\datatable \pgfplotsretval}
    }

    \addplot[mark=none,color=red,thick,solid,forget plot]
    table [x index={0},y index={1}] {data.out};


    \addplot[scatter,scatter src=\thisrowno{2},mark=square*,color=blue,thick,solid,forget plot]
    table [x expr=\thisrowno{0}+1,y index={1}] {data.out};

    \nextgroupplot

    \addlegendimage{mark=*,mark options={draw=black,fill=white},color=red,thick,solid}
    \addlegendentry{$a$}
    \addlegendimage{mark=square*,mark options={draw=black,fill=white},color=blue,thick,solid}
    \addlegendentry{$b$}
    \addplot[scatter,scatter src=\thisrowno{2},mark=*,color=red,thick,solid]
    table [x index={0},y index={1}] {data.out};

    \addplot[scatter,scatter src=\thisrowno{2},mark=square*,color=blue,thick,solid]
    table [x expr=\thisrowno{0}+1,y index={1}] {data.out};


  \end{groupplot}


  \node  at ($(myplot c1r1.south)!0.5!(myplot c2r1.south)$)
  [inner sep=0pt,anchor=north, yshift=-5ex]
  {\ref{grouplegend}};


\end{tikzpicture}

\end{document}

The idea is copied from Jake's answer here.