documentclass{article}
usepackage{listings}
usepackage{xcolor}

lstdefinelanguage{bluelang}{
  morekeywords=[1]{foo,bar}
}

lstdefinelanguage{redlang}{
  morekeywords=[2]{foo,baz}
}%

lstset{%
  alsolanguage=bluelang,
  alsolanguage=redlang,
  keywordstyle={[1]color{blue}},
  keywordstyle={[2]color{red}},
  basicstyle=ttfamily,
}%

begin{document}
begin{lstlisting}
foo bar baz
end{lstlisting}
%
lstset{deletekeywords=[1]{foo}}
%
begin{lstlisting}
foo bar baz
end{lstlisting}
%
lstset{morekeywords=[2]{foo}}
%
begin{lstlisting}
foo bar baz
end{lstlisting}
end{document}