\documentclass[xcolor=dvipsnames]{beamer}
\usecolortheme[named=blue]{structure}
\usepackage[tikz]{bclogo}
%------------------------\AtBeginSubsection[]----------------------
\AtBeginSubsection[]{%
\begin{frame}<beamer>[noframenumbering,plain]{Outline}
\transsplitverticalout
\tableofcontents[currentsection,currentsubsection]
\end{frame}
}
\usetheme{Madrid}
\begin{document}
\begin{frame}{}
\begin{bclogo}[arrondi=0.1, logo=\bcquestion, couleurBarre=Maroon]
{Question}
Why
$$r_e(\mathbf{T})\geq r(\mathbf{T}) \;?$$
\end{bclogo}
\end{frame}
\end{document}
After compilation I get
However I want to change the color of the word question and to underline it as the following picture shows




$$...$$should be avoided, so it would be better to use\[r_e(\mathbf{T})\geq r(\mathbf{T}) \;?\], see https://tex.stackexchange.com/q/503/35864. – moewe Dec 17 '18 at 15:29$$...$$will give different spacing than\[...\](and people usually prefer the spacing\[...\]gives).\[...\]also works together with thefleqnoption and provides for better placement of\qedhere. You are of course free to ignore the advice not to use$$...$$, but I think the comment has a place here to warn other people who might be more susceptible to the arguments in the linked answer. – moewe Dec 17 '18 at 15:47