# feynmf: colour flow lines

I would like to add colour flow lines to Feynman diagrams as in this example:

Is there a convenient way to have such a functionality?

The code is the following:

\documentclass[class=minimal,border=20pt]{standalone}
\usepackage{feynmf}
\begin{document}
\begin{fmffile}{diagram}
\begin{fmfgraph*}(150, 150)
\fmfleft{i}
\fmfright{o1,o2}
\fmf{fermion}{i,v,o1}
\fmf{gluon}{v,o2}
\end{fmfgraph*}
\end{fmffile}
\end{document}


This is a code which produces something rather similar to what you want:

 \documentclass[class=minimal,border=20pt]{standalone}
\usepackage{feynmp}

\newcommand{\LFermionFlow}[7][X]
% Usage: see \LFermionFlow[Deformation]{Node1}{Node2}{Length}{Shift x}{Shift y}{color}
{
\fmffreeze
\fmf{phantom}{#2,#2FFNode#3}
\fmf{phantom,tension=#4}{#2FFNode#3,#3FFNode#2}
\fmf{phantom}{#3FFNode#2,#3}
\fmffreeze
\fmfshift{#5,#6}{#2FFNode#3,#3FFNode#2}
\fmffreeze
\if X#1 {
\fmf{fermion,width=0.5,fore=#7,left=0.3}{#2FFNode#3,#3FFNode#2}
}
\else
{
\fmf{fermion,width=0.5,fore=#7,left=#1}{#2FFNode#3,#3FFNode#2}
}
\fi
}

\newcommand{\RFermionFlow}[7][X]
% Usage: see \RFermionFlow[Deformation]{Node1}{Node2}{Length}{Shift x}{Shift y}{color}
{
\fmffreeze
\fmf{phantom}{#2,#2FFNode#3}
\fmf{phantom,tension=#4}{#2FFNode#3,#3FFNode#2}
\fmf{phantom}{#3FFNode#2,#3}
\fmffreeze
\fmfshift{#5,#6}{#2FFNode#3,#3FFNode#2}
\fmffreeze
\if X#1 {
\fmf{fermion,width=0.5,fore=#7,right=0.3}{#2FFNode#3,#3FFNode#2}
}
\else
{
\fmf{fermion,width=0.5,fore=#7,right=#1}{#2FFNode#3,#3FFNode#2}
}
\fi
}
\begin{document}
\begin{fmffile}{diagram}
\begin{fmfgraph*}(150, 150)
\fmfleft{i}
\fmfright{o1,o2}
\fmf{fermion,label=$~$}{i,v,o1}
\fmf{gluon,label=$~$}{v,o2}
\fmffreeze
\RFermionFlow{i}{o2}{0.5}{0.5}{0}{blue}
\RFermionFlow{o2}{o1}{0.5}{0.5}{0}{red}
\end{fmfgraph*}
\end{fmffile}
\end{document}


I removed the corners from the flow lines.

Of course, here this is not a fermion flow (for which the command originally was designed), but it can perhaps also serve as a `color flow'.