How to turn off drawing node border in PGF?

Use \node[draw=none]. From the TikZ manual (section 15.3 “Drawing a Path” in v2.10),

If the special color name none is given, this option causes drawing to be “switched off.” This is useful if a style has previously switched on drawing and you locally wish to undo this effect.

I think you need to draw the lines manually:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{matrix}
\begin{document}
\begin{tikzpicture}
\matrix(m)[matrix of nodes,nodes={minimum width=2em,font=\LARGE}]
{
  & R & & \\
R & B & G & W\\
  & R & & \\
};
\draw (m-3-2.south west) rectangle (m-1-2.north east);
\draw (m-2-1.south west) rectangle (m-2-4.north east);
\draw (m-2-3.south east) -- (m-2-3.north east);
\end{tikzpicture}
\end{document}

TikZ's matrix library is apparently necessary to access the nodes in a matrix.

Now you can take that and make it the replacement text of a macro so you won't have to type it over and over.