\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\usepackage{color}
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\begin{document}
\begin{equation*}
  C(s) = \frac{k_d s^2 + k_p s + k_i}{s}
  = \frac{k_i}{s} \frac{s^2 + 2\zeta {\color{blue} a} s + a^2}{a^2}
\end{equation*}
\end{document}