\documentclass[12pt]{article}
\usepackage{amssymb,amsmath,amscd,concmath,amsfonts}
\usepackage{color}
\pagestyle{empty}
\begin{document}
\begin{equation*}
  B(t) = \left. \frac{\partial F}{\partial v} \right|_{(x_d(t), u_\text{ff}(t){\color{blue})}}.
\end{equation*}
\end{document}