If you use PNG or another bitmap file, you can preprocess your file with Imagemagick.
convert -negate in.png out.png
This also works for PDFs. However, the vector data is rasterised and that's not what you aim for.
Edit:
You can preprocess the file with inkscape. Load the pdf file, then select Extensions → Colour → Negative, and the paper size of the size of your imported PDF. This yields a non-rasterised, inverted PDF.
To import a slide from another beamer presentation as a "picture", you can use pdfpages. You'll have to \setbeamercolor{background canvas}{bg=} as explained in this answer because only a white frame will be shown otherwise:
\documentclass{beamer}
\usepackage{pdfpages}
\begin{document}
\begin{frame}
Content
\end{frame}
{
\setbeamercolor{background canvas}{bg=}
\includepdf[pages=3]{filea.pdf}
}
\end{document}
However, if you use head- or footlines or even a different theme in your existing presentation, this will probably not fit snugly. In this case, a better solution would be to actually import the source code of the existing presentation, which can be done using the docmute package in combination with the \againframe command:
Suppose this is your existing presentation filea.tex: You need to give the slides you want to import a name using the label option of the frame environment, like this:
\documentclass{beamer}
\begin{document}
\begin{frame}[label=myframe]
Frame to be included
\end{frame}
\end{document}
Then you can use this code in your new presentation:
\documentclass{beamer}
\usepackage{docmute}
\makeatletter
\newcommand*{\loadpresentation}[1]{{\beamer@inlecturefalse\input{#1}}}
\makeatother
\begin{document}
\loadpresentation{filea.tex}
\begin{frame}
The new presentation
\end{frame}
\againframe{myframe}
\end{document}
Issuing \loadpresentation{filea.tex} imports the frames from your existing presentation without displaying them. You can insert them wherever needed using \againframe with the label you chose in filea.tex. The command \loadpresentation should be used someplace after \begin{document} (but before you actually include a frame from this presentation, of course).
This works roughly the same as if you'd actually copy the source code of the frame from the existing presentation, so things like overlays etc. are taken over.
Best Answer
Pass the option
grayto the packagexcolorEDIT:
blackandwhite wrote that it doesn't have any effect. Here is a full minimal working example which works well: