\documentclass[12pt]{article}
\usepackage{pstricks}
\SpecialCoor
\usepackage{pst-xkey}

\makeatletter
\define@key[psset]{}{transpalpha}{\pst@checknum{#1}\pstranspalpha}
\psset{transpalpha=0.5}
%
\def\psfs@transp{%
  \addto@pscode{/Normal .setblendmode \pstranspalpha .setshapealpha}
  \psfs@solid%
}
\makeatother

\pagestyle{empty}
\parindent=0pt
\begin{document}

\psset{linestyle=none,fillstyle=transp}
\begin{pspicture}(-5,-5)(5,5)
  \rput(0,1){\Huge\textbf{PSTricks}}
  \pscircle[fillcolor=green](2;135){2cm}
  \pscircle[fillcolor=red](2;45){2cm}
  \pscircle[fillcolor=blue](1;-90){2cm}
\end{pspicture}

\end{document}
