\tikzset{My Rectangle/.style={rectangle, draw=brown, fill=yellow, thick,
prefix after command= {\pgfextra{\tikzset{every label/.style={blue}}}}}}
\begin{tikzpicture}
\node [rectangle, label=black text] at (1,0) {$z$};
\node [My Rectangle, label={blue text} ] at (3,0) {$z$};
\node [rectangle, label={blue text} ] at (5,0) {$z$};
\end{tikzpicture}
\end{document}