documentclass[10pt]{beamer}
usetheme{metropolis}
begin{document}
begin{frame}{Test}
centering
includegraphics{xxx.pdf}
end{frame}
end{document}

convert xxx.pdf xxxx.pdf

documentclass[10pt]{beamer}
usetheme{metropolis}

definecolor{myback}{RGB}{250, 250, 250}

begin{document}

begin{frame}{Test}
centering
{color{myback}includegraphics{xxx.pdf}}
end{frame}
end{document}