\documentclass[pstricks]{standalone}

\psset
{
	fillstyle=solid,
	fillcolor=cyan,% does NOT affect pscustom
	linewidth=2pt,
	linecolor=red,
}

\begin{document}

% pscustom cannot take the global fillcolor.
\begin{pspicture}(3,3)
\pscustom{\psframe(1,1)(2,2)}
\end{pspicture}

% so local fillcolor must be explicitly specified.
\begin{pspicture}(3,3)
\pscustom[fillcolor=orange]{\psframe(1,1)(2,2)}
\end{pspicture}

\end{document}

\documentclass[pstricks]{standalone}

%\makeatletter
%\def\foo#1\psset#2#3!!{\def\@begindocumentaaa{#1#3}}
%\expandafter\foo\@begindocumentaaa!!
%\makeatother

\usepackage{etoolbox}
\expandafter\patchcmd\csname @begindocumenthook\endcsname{\psset{fillcolor=white}}{}{}{}

\psset
{
	fillstyle=solid,
	fillcolor=cyan,% does NOT affect pscustom
	linewidth=2pt,
	linecolor=red,
}

\begin{document}

% pscustom cannot take the global fillcolor.
\begin{pspicture}(3,3)
\pscustom{\psframe(1,1)(2,2)}
\end{pspicture}