To change the text color in the Beamer class, you can use the following example.
First, we set the background color to gray to ensure that white text is visible against it. This is done using \setbeamercolor{background canvas}{bg=black!50}. Then, we set the color of the "normal" text to white using \setbeamercolor{normal text}{fg=white}. It's important to note that without \usebeamercolor*{normal text} following these commands, the changes won't take effect.
Before the second frame, we reset to the default settings.
\documentclass{beamer}
\begin{document}
\setbeamercolor{background canvas}{bg=black!50}
\setbeamercolor{normal text}{fg=white}\usebeamercolor*{normal text}
\begin{frame}
TEST
\end{frame}
\setbeamercolor{background canvas}{bg=white,fg=black}
\setbeamercolor{normal text}{fg=black}\usebeamercolor*{normal text}
\begin{frame}
TEST 2
\end{frame}
\end{document}
You can use the same method with other colors as well.