Document Classes ⇒ Problem with colors in beamer-class

[Golden_Eagle]

Hello,

I am using Miktex 2.7 with the beamer package. I want to change the background-color and the font-color (including a caption) for a single frame. For the background, the command \setbeamercolor is working, but not for the text. I tried

\setbeamercolor{normal text}{bg=yellow,fg=yellow}

I have searched for the problem for about two hours, but didn't find it, the package documentation did not help, either.

Thank you for your help!

Best regards,
Johannes

Minimal example:
----
\documentclass[12pt]{beamer}

\mode<presentation>
\usetheme{Boadilla}
\useoutertheme{infolines}
\usecolortheme{whale}

\usepackage{ngerman}
\usepackage[latin1]{inputenc} %für Umlaute
\usepackage{amsmath}
\usepackage{amsthm}
\usepackage{array}
\usepackage{enumerate}
\usepackage{hhline}
\usepackage{graphicx}
\usepackage[font=scriptsize,labelfont=scriptsize,listformat=empty,figurename=Abbildung]{caption}
\usepackage{microtype}
\usepackage{type1cm}

\sloppy

\begin{document}

{
\setbeamercolor{background canvas}{bg=black}
\setbeamercolor{normal text}{bg=yellow}

\begin{frame}
\begin{figure}
\centering
\includegraphics[width=1\textwidth]{bilder/gcenter_xray_rgb.pdf}
\caption{<text>}
\end{figure}
<more text>
\end{frame}
}

\end{document}
---