Underbrace in equation

Question

What is wrong with underbrace please, there is no text below after the compilation

\begin{equation}
\underbrace{{\color{red}I_j (x)} \ast \Delta_j(x,t_l,{\color{red}p})}_{\text{I_j(x)(x-v_j(t,p))}} \mathrm{d}x
\end{equation}

Answer 1

\text is when the subscript is textual.

You should also prefer \textcolor and brace the whole \underbrace construct.

\documentclass{article}
\usepackage{amsmath}
\usepackage{xcolor}

\begin{document}

\begin{equation}
 {% make the \underbrace an ordinary atom
  \underbrace{\textcolor{red}{I_j (x)} \ast \Delta_j(x,t_l,\textcolor{red}{p})}%
    _{I_j(x)(x-v_j(t,p))}
 }\,\mathrm{d}x
\end{equation}

\end{document}

Why preferring \textcolor? Because you can redefine it to preserve the nature of math atoms:

\documentclass{article}
\usepackage{amsmath}
\usepackage{xcolor}

\makeatletter
\renewcommand{\@textcolor}[3]{%
  \relax
  \ifmmode
    \begingroup\color#1{#2}#3\endgroup
  \else
    \protect\leavevmode{\color#1{#2}#3}%
  \fi
}
\makeatother

\begin{document}

$a\textcolor{red}{=}b$

$a{\color{red}=}b$

\end{document}