Beamer’s normal text is black on white background. The following command in the preamble changes all normal text to purple:
\setbeamercolor{normal text}{fg=purple}
The fg in command above sets the normal text’s foreground color.
It is also possible to set the normal text’s background color.
The background color of a slide is inherited from the normal text’s
background, therefore changing the normal text’s background color amounts
to changing the slide’s background color. See Setting the background to a solid color for
an illustration.
Remark: Normal text is the text in the main body of a slide. A slide’s header and footer are not normal text. The colors of those elements are controlled by the structure color. For instance, the following command changes slide titles (and several other things) to purple:
\setbeamercolor{structure}{fg=purple}
See the sections An image for a background and The structure color for illustrations.