Code: Select all
\documentclass[12pt,letterpaper]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{mathtools}
\usepackage{xcolor}
\begin{document}
A simple test :
\begin{align*}
A = \color[gray]{0.4}{\underbracket[0.5pt][4pt]{\color{black} BCD}_{pq - st}}\color{black} \, EFG &= KLM \\[6pt]
&= ABC \color[gray]{0.4}{\underbracket[0.5pt][4pt]{\color{black} PQ}_{r s + u v}}\color{black} - MN.
\end{align*}
\end{document}
Code: Select all
\undercom{<color>}{<math item>}{<comment>}
(sorry for my bad English, I hope the query is clear).