How to color a tag in a math equation?
\documentclass{article}
\usepackage{amsmath,amssymb}
\usepackage{mathtools}
\usepackage{xcolor}
\newtagform{red}{\color{red}(}{)}
\usetagform{red}
\begin{document}
\begin{align*}
\color{red} \sum_{n=1}^\infty\dfrac{1}{n^2}=\dfrac{\pi^2}{6}.\tag{E1}
\end{align*}
\end{document}
You can redefine the tag
Hope this helps!
CODE with newcommand
\documentclass{article}
\usepackage{amsmath,amssymb}
\usepackage{xcolor}
\newcommand{\TAG}[1]{\tag*{\textcolor{red}{(#1)}}}
\begin{document}
This is my favorite equality:
\begin{align*}
\color{red}\sum_{n=1}^\infty\dfrac{1}{n^2}=\dfrac{\pi^2}{6}.\TAG{E1}
\end{align*}
\end{document}
CODE based on the comments by daleif and Mico
\documentclass{article}
\usepackage{amsmath,amssymb}
\usepackage{xcolor}
\begin{document}
This is my favorite equality:
\begin{align*}
\color{red}\sum_{n=1}^\infty\dfrac{1}{n^2}=\dfrac{\pi^2}{6}.
\tag*{\textcolor{red}{(E1)}}
\end{align*}
\end{document}