color only a cell of a table

Here is a minimal working example (MWE) showing how it can be obtained using xcolor's table option - this loads colortbl:

\documentclass{article}
\usepackage[table]{xcolor}% http://ctan.org/pkg/xcolor
\begin{document}
\begin{tabular}{l|c|r}
  \hline
  Some & \cellcolor{blue!25}coloured & contents \\
  \hline
\end{tabular}
\end{document}

The line choices in the above MWE emphasizes the tabular structure. However, for aesthetically pleasing table construction, consider using booktabs.

Use the colortbl package and its macro \cellcolor{<color>}.

\documentclass{article}
%% Set some local commands and colors
\usepackage{xcolor,colortbl}
\definecolor{green}{rgb}{0.1,0.1,0.1}
\newcommand{\done}{\cellcolor{teal}done}  %{0.9}
\newcommand{\hcyan}[1]{{\color{teal} #1}}
\begin{document}
\begin{tabular}{ll}
\done & other\\
\end{tabular}
\end{document}