% A simple cycle
% Author : Jerome Tremblay
\documentclass{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
    \node[shape=circle,draw=blue] (A) at (0,0) {$v_1$};
    \node[shape=circle,draw=red]  (B) at (2,2) {$v_2$};
    \node[shape=circle,draw=blue] (C) at (0,4) {$v_3$};

    \path [-](A) edge node[left] {} (B);
    \path [-](B) edge node[left] {} (C);
\end{tikzpicture}
\end{document}