\documentclass[varwidth=true, border=2pt]{standalone}
\usepackage{tikz}
\usetikzlibrary{arrows,positioning}

\begin{document}
  \tikzstyle{vertexs}=[draw,fill=black,circle,minimum size=4pt,inner sep=0pt]

  \begin{tikzpicture}
    \foreach \x in {0,1,2}
    \foreach \y in {0,1,2}{
      \node (a)[vertexs] at (\y,0) {};
      \node (b)[vertexs] at (\x,1) {};
        \draw (a) -- (b);
    }
    \draw[green,ultra thick] (1,1) -- (0,0);
    \node (b)[vertexs,red] at (1,1) {};
    \node (b)[vertexs,red] at (0,0) {};
  \end{tikzpicture}
\end{document}