\documentclass{beamer}

%\usetheme{Dresden}
%\usecolortheme{dolphin}
%\useoutertheme{miniframes}

%\usepackage{amsmath,amssymb,amsthm,commath,mathtools}
%\usepackage{enumerate}
%\usepackage{subcaption}
%\usepackage{graphicx}
%\usepackage{adjustbox}

\usepackage{adjustbox}

\usepackage{color,soul}
\definecolor{darkblue}{rgb}{0,0,0.5}

\setulcolor{red}

% Source: https://tex.stackexchange.com/questions/41683/why-is-it-that-coloring-in-soul-in-beamer-is-not-visible
\let\UL\ul
\makeatletter
\renewcommand\ul{
	\let\set@color\beamerorig@set@color
	\let\reset@color\beamerorig@reset@color
	\UL
}

\makeatother

\begin{document}
	\begin{frame}
		\ul{Test}
	\end{frame}
	
\end{document}