I use \documentclass{scrbook} with \usepackage[Bjornstrup]{fncychap}.

How can I style the partpage like the chapterpage in the Bjornstrup-style?

Plus this I want to change the grey colour of the box from Bjornstrup in another one. How is this possible?

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Welcome to TeX.sx! Usually, we don't put a greeting or a "thank you" in our posts. While this might seem strange at first, it is not a sign of lack of politeness, but rather part of our trying to keep everything very concise. Upvoting is the preferred way here to say "thank you" to users who helped you. A tip: You can use backticks  to mark your inline code as I did in my edit. –  Torbjørn T. Dec 2 '11 at 14:44

Here's one possible solution: I redefined \partformat and the internal command \@part (from scrbook.cls) to imitate the Bjornstrup style for chapters; I also redefined \DOCH, \DOTI and \DOTIS as defined by Bjornstrup to allow a variable color. You can now control the background color for parts and chapters simply redefining (through \colorlet) the colors partbgcolor, and chapbgcolor; the color for the numbers used can similarly be changed using partnumcolor and chapnumcolor:

\documentclass{scrbook}
\usepackage[Bjornstrup]{fncychap}
\usepackage{xcolor}

\colorlet{partbgcolor}{gray!30}% shaded background color for parts
\colorlet{partnumcolor}{gray}% color for numbers in parts
\colorlet{chapbgcolor}{gray!30}% shaded background color for chapters
\colorlet{chapnumcolor}{gray}% color for numbers in chapters

\renewcommand*\partformat{%
\fontsize{76}{80}\usefont{T1}{pzc}{m}{n}\selectfont%
\hfill\textcolor{partnumcolor}{\thepart}}

\makeatletter
\renewcommand*{\@part}{}
\def\@part[#1]#2{%
\ifnum \c@secnumdepth >-2\relax
\refstepcounter{part}%
\@maybeautodot\thepart%
\else
\fi
\begingroup
\setparsizes{\z@}{\z@}{\z@\@plus 1fil}\par@updaterelative
\raggedpart
\interlinepenalty \@M
\normalfont\sectfont\nobreak
\setlength\fboxsep{0pt}
\colorbox{partbgcolor}{\rule{0pt}{40pt}%
\makebox[\linewidth]{%
\begin{minipage}{\dimexpr\linewidth+20pt\relax}
\ifnum \c@secnumdepth >-2\relax
\vskip-25pt
\size@partnumber{\partformat}%
\fi      %
\vskip\baselineskip
\hspace*{\dimexpr\myhi+10pt\relax}%
\parbox{\dimexpr\linewidth-2\myhi-20pt\relax}{\raggedleft\LARGE#2\strut}%
\hspace*{\myhi}\par\medskip%
\end{minipage}%
}%
}%
\partmark{#1}\par
\endgroup
\@endpart
}

\renewcommand\DOCH{%
\settowidth{\py}{\CNoV\thechapter}
\fboxsep=0pt%
\colorbox{chapbgcolor}{\rule{0pt}{40pt}\parbox[b]{\textwidth}{\hfill}}%
\kern-\py\raise20pt%
\hbox{\color{chapnumcolor}\CNoV\thechapter}\\%
}

\renewcommand\DOTI[1]{%
\nointerlineskip\raggedright%
\fboxsep=\myhi%
\vskip-1ex%
\colorbox{chapbgcolor}{\parbox[t]{\mylen}{\CTV\FmTi{#1}}}\par\nobreak%
\vskip 40pt%
}

\renewcommand\DOTIS[1]{%
\fboxsep=0pt
\colorbox{chapbgcolor}{\rule{0pt}{40pt}\parbox[b]{\textwidth}{\hfill}}\\%
\nointerlineskip\raggedright%
\fboxsep=\myhi%
\colorbox{chapbgcolor}{\parbox[t]{\mylen}{\CTV\FmTi{#1}}}\par\nobreak%
\vskip 40pt%
}
\makeatother

\begin{document}

\part{Test Part}
\chapter{Test Chapter}

\end{document}


Here's how parts will now look like:

And using

\colorlet{partbgcolor}{LightSkyBlue}% shaded background color for parts
\colorlet{partnumcolor}{NavyBlue}% color for numbers in parts
`

you will get:

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WOW, Thankxxxxx again!!!!! –  Bjornstrup Dec 7 '11 at 9:17
@Bjornstrup: You're welcome! Since my response seems to answer your question, please consider marking it as ‘Accepted’ by clicking on the tickmark below its vote count. –  Gonzalo Medina Dec 7 '11 at 23:19