\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\usepackage{color}
\pagestyle{empty}
\begin{document}
\begin{equation*}
  k_i=2 \zeta \omega_0 (1-{\color{blue} s_m}).
\end{equation*}
\end{document}