Why is it so? My MWE:
\documentclass{article}
\usepackage{tabularx}
\usepackage[table]{xcolor}
\begin{document}
\begin{table}[H]
\caption{Title.}\label{title}
\begin{tabularx}{\linewidth}{|p{5.5cm}|X|}
\hline
\color{red}{\textbf{Column 1}} & \color{red}{\textbf{Column 2}} \\
\hline
A & a \\
\hline
B & b \\
\hline
C & c \\
\hline
\end{tabularx}
\end{table}
\end{document}
The result is as follows

If color is black, then the height is as I expect:

\textcolorinstead ofcolor. – Oct 22 '13 at 11:13