Programmatically setting column style of a matrix in TikZ

You can expand the variables by using an \edef to define a new command:

\documentclass{article}
\usepackage{pgfplots}
\usetikzlibrary{matrix}

\begin{document}

\begin{tikzpicture}

\tikzstyle{mymatrix}=[draw]  
\foreach \c [count=\i] in {red, green, blue} {
    \globaldefs=1 
    \edef\dotikzset{
        \noexpand\tikzset{
            mymatrix/.append style={
                column \i/.style={
                    nodes={fill=\c}
                }
            }
        }
    }
    \dotikzset
}

\matrix (m) [matrix of nodes, mymatrix] {
 1 & 2 & 3 & 4 \\
 5 & 6 & 8 & 8 \\ };

\end{tikzpicture}

\end{document}

Jake was faster but here is something I learned from egreg in this question :

\documentclass{article}
\usepackage{etoolbox}
\usepackage{tikz}
\usetikzlibrary{matrix}

\let\mystyle\empty
\newcommand{\populatestyle}{%
  \foreach \c [count=\i] in {red, green, blue} {
    \begingroup\edef\x{\endgroup
       \noexpand\gappto\noexpand\mystyle{column \i/.style={nodes={fill=\c}},}}\x
    }%
}
\populatestyle

\tikzset{mymatrix/.estyle={\mystyle}}

\begin{document}
\begin{tikzpicture}
\matrix (m) [matrix of nodes,mymatrix] {
 1 & 2 & 3 & 4 \\
  5 & 6 & 7 & 8 \\ };
\end{tikzpicture}
\end{document}