\documentclass{article}

\usepackage{pstricks}

\begin{document}

\begin{pspicture}(5,5)
  \psset{fillstyle=solid,framearc=.3}
  \psframebox[fillcolor={rgb:red,1;green,2;blue,3}]{\textcolor{white}{TEST}}
  \psframebox[fillcolor={rgb:red,100;green,200;blue,300}]{\textcolor{white}{TEST}}
  \psframebox[fillcolor={rgb:red,50;green,100;blue,150}]{\textcolor{white}{TEST}}
\end{pspicture}

\end{document}​