\documentclass[12pt]{article}

\usepackage{color,  amssymb}
\usepackage{hyperref}
\usepackage{tikz}
\usepackage{tikz-cd}

\newtheorem{thm}{Theorem}

\begin{document}

\begin{center}
\bf
Example of a Commutative Diagram
\end{center}

\begin{center}

\begin{tikzcd}
A \arrow[r, "\phi"] \arrow[d, red]
& B \arrow[d, "\psi" red] \\
C \arrow[r, red, "\eta" blue]
& D
\end{tikzcd}

\end{center}

See \url{http://ctan.math.washington.edu/tex-archive/graphics/pgf/contrib/tikz-cd/tikz-cd-doc.pdf}


\end{document}



