\begin{center}
\setlength{\unitlength}{0.75cm}
\referencesystem(0,0)(1,-1)(1,1)
\begin{Picture}(-2.5,-2.5)(2.5,2.5)
\cartesiangrid(-2,-2)(2,2)
\pictcolor{red}
\put(1,-1){\circle*{0.25}}
\pictcolor{blue}
\Put(1,-1){\circle*{0.25}}
\end{Picture}
\end{center}
